Semiconductor device

ABSTRACT

It is an object to provide a semiconductor device in which power consumption can be reduced. It is another object to provide a highly reliable semiconductor device using a programming cell, such as a programmable logic device (PLD). In accordance with a change in a configuration of connections between basic blocks, power supply voltage furnishing to the basic blocks is changed. That is, when the structure of connections between the basic blocks is such that a basic block does not contribute to a circuit, the supply of the power supply voltage to this basic block is stopped. Further, the supply of the power supply voltage to the basic blocks is controlled using a programming cell formed using a field effect transistor whose channel formation region is formed using an oxide semiconductor, the field effect transistor having extremely low off-state current or extremely low leakage current.

TECHNICAL FIELD

The present invention relates to a semiconductor device including asemiconductor element using an oxide semiconductor.

BACKGROUND ART

In recent years, a metal oxide exhibiting semiconductor characteristicsand called oxide semiconductor has attracted attention as a novelsemiconductor material having higher mobility and higher reliabilitythan amorphous silicon, and having uniform element characteristics.Metal oxides are used for various applications. For example, indiumoxide is a well-known metal oxide and used as a material of atransparent electrode included in a liquid crystal display device or thelike. Examples of such metal oxides having semiconductor characteristicsinclude tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thinfilm transistors whose channel formation region is formed using such ametal oxide having semiconductor characteristics are known (PatentDocuments 1 and 2).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

Conventionally, in semiconductor integrated circuits such as a normalcentral processing unit (CPU), the circuits are designed in advance andcannot be changed after manufacturing. In contrast, in a semiconductorintegrated circuit called a programmable logic device (PLD), a logiccircuit is formed using basic block units having an adequate structureand interconnections between the basic block units can be changed aftermanufacturing. Therefore, the PLD has high versatility because a circuitconfiguration can be changed by a user, and the time and cost spent ondesign and development of a circuit can be greatly reduced.

The PLD includes a complex PLD (CPLD), a field programmable gate array(FPGA), and the like. When the program of the circuit configuration canbe rewritten without limitation, the circuit configuration of any of theabove PLDs is set in accordance with a structure of connections betweenbasic blocks which is stored in a semiconductor memory such as an EEPROMor an SRAM. Depending on the circuit configuration which is programmed,there can be a case where a basic block does not contribute to thecircuit configuration. In particular, as a PLD has a larger size andhigher versatility, the number of basic blocks increases; since thecircuit configuration is customized for a specific use, the number ofbasic blocks which do not contribute to the circuit configurationincreases.

However, power supply voltage is supplied also to the basic blocks whichdo not contribute to the circuit configuration in the PLD. Therefore, inthe basic blocks, various capacitance including parasitic capacitanceare charged and discharged wastefully, so that power is consumed. Whenthe power supply voltage is supplied, unnecessary power consumptionoccurs in the basic blocks due to leakage current or off-state current.For example, in the case where an inverter is formed using a CMOS,although, ideally, power is not consumed in a steady state, power isconsumed in practice due to leakage current which flows to a gateinsulating film or to off-state current which flows between a sourceelectrode and a drain electrode. In the case where a CMOS inverter isformed using bulk silicon, an off-state current of approximately 1 pA isgenerated at a room temperature with a power supply voltage ofapproximately 3 V. When high integration of the PLD is achieved, powerconsumption is expected to be increased due to miniaturization of asemiconductor element or an increase in the number of elements.

In the case of using a rewritable PLD, the EEPROM or the SRAM isgenerally used as a programming cell for storing a structure ofconnections. However, since the EEPROM writes and erases data utilizingthe principle of tunneling current, deterioration of an insulating filmeasily occurs. Accordingly, in practice, the number of times ofrewriting data is not infinite, and the upper limit of the number ofrewritings is approximately tens of thousands of times to hundreds ofthousands of times. In addition, an absolute value of the voltageapplied to the EEPROM when data is written and when data is erased is ashigh as around 20 V. Therefore, power consumption easily increases whendata is written and when data is erased. In addition, a redundantcircuit design for covering the size of the above operating voltage isnecessary.

On the other hand, the SRAM always needs the supply of power supplyvoltage to hold data. Accordingly, power is consumed even in a steadystate while the power supply voltage is supplied, as described above;therefore, if the number of programming cells using the SRAM increaseswith high integration, power consumption of a semiconductor deviceincreases.

In view of the above problems, it is an object of the present inventionto provide a semiconductor device which can reduce power consumption. Itis another object of the present invention to provide a highly reliablesemiconductor device using a programming cell.

In a semiconductor device according to one embodiment of the presentinvention, in accordance with changing of a structure of connectionsbetween basic blocks, presence or absence of the supply of the powersupply voltage to the basic blocks is changed. That is, when changing ofthe structure of the connection between the basic blocks generates abasic block which does not contribute the circuit configuration, thesupply of the power supply voltage to the basic block in question isstopped.

In one embodiment of the present invention, the supply of the powersupply voltage to the basic blocks is controlled using a programmingcell formed using an insulated gate field effect transistor (hereinaftersimply referred to as a transistor) with extremely low off-state currentor extremely low leakage current. The band gap of the above transistoris wider than that of a silicon semiconductor, and a channel formationregion of the transistor includes a semiconductor material whoseintrinsic carrier density is lower than that of silicon. A semiconductormaterial having such characteristics is included in a channel formationregion, so that a transistor with extremely low off-state current can berealized. As examples of such a semiconductor material, an oxidesemiconductor having a band gap which is approximately three times aslarge as that of silicon can be given.

In one embodiment of the present invention, the transistor withextremely low off-state current or extremely low leakage current is usedfor a programming cell for controlling a connection between the basicblocks.

Unless otherwise specified, in this specification, in the case of ann-channel transistor, an off-state current is a current which flowsbetween a source electrode and a drain electrode when a potential of thedrain electrode is higher than that of the source electrode or that of agate electrode while a potential of the gate electrode is less than orequal to zero when a reference potential is the potential of the sourceelectrode. Alternatively, in this specification, in the case of ap-channel transistor, an off-state current is current which flowsbetween a source electrode and a drain electrode when a potential of thedrain electrode is lower than that of the source electrode or that of agate electrode while the potential of the gate electrode is greater thanor equal to zero when a reference potential is the potential of thesource electrode.

Specifically, the above programming cell includes at least a firsttransistor functioning as a switching element for controlling aconnection between two nodes and a second transistor for controlling thesupply of a potential to a gate electrode of the above first transistor.The second transistor includes a channel formation region formed using asemiconductor material, such as an oxide semiconductor, which has a bandgap approximately three times as large as that of silicon, and anoff-state current or a leakage current extremely low. Accordingly, thevoltage (gate voltage) between the gate electrode and the sourceelectrode of the first transistor is held for a long time by the secondtransistor which has extremely low off-state current or extremely lowleakage current.

An oxide semiconductor is a metal oxide exhibiting semiconductorcharacteristics, which includes a high mobility which is almost the sameas that of microcrystalline silicon or polycrystalline silicon, and auniformity in characteristics of elements which is almost the same asthat of amorphous silicon. An oxide semiconductor highly-purified (apurified OS) by reduction in concentration of impurities such asmoisture or hydrogen, which serves as electron donors (donors), is anintrinsic semiconductor (an i-type semiconductor) or a substantiallyintrinsic semiconductor. Therefore, a transistor including the aboveoxide semiconductor has a characteristic of extremely low off-statecurrent or extremely low leakage current. Specifically, the hydrogenconcentration in the highly-purified oxide semiconductor which ismeasured by secondary ion mass spectrometry (SIMS) is less than or equalto 5×10¹⁹/cm³, preferably less than or equal to 5×10¹⁸/cm³, morepreferably less than or equal to 5×10¹⁷/cm³, still more preferably lessthan or equal to 1×10¹⁶/cm³. In addition, the carrier density of theoxide semiconductor film, which is measured by Hall effect measurement,is less than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, morepreferably less than 1×10¹¹/cm³. Furthermore, the band gap of the oxidesemiconductor is greater than or equal to 2 eV, preferably greater thanor equal to 2.5 eV, more preferably greater than or equal to 3 eV. Withthe use of the oxide semiconductor film which is highly purified bysufficiently reducing the concentration of impurities such as moistureor hydrogen, off-state current or leakage current of the transistor canbe reduced.

The analysis of the hydrogen concentration in the oxide semiconductorfilm is described here. It is known that it is, in principle, difficultto obtain data accurately in the proximity of a surface of a sample orin the proximity of an interface between stacked films formed usingdifferent materials by the SIMS analysis principle. Thus, in the casewhere distributions of the hydrogen concentrations of the film areanalyzed by SIMS in thickness directions, when, in a region of the filmunder study, the value does not vary greatly and an almost constantvalue can be obtained, an average value is accepted as the hydrogenconcentration. Further, in the case where the thickness of the filmunder study is small, a region with an almost constant value cansometimes not be obtained due to the influence of the hydrogenconcentration in adjacent films. In that case, a local maximum value ora local minimum value of the hydrogen concentration in a region of thefilm is accepted as the hydrogen concentration in the film. Furthermore,in the case where a mountain-shaped peak having the local maximum valueand a valley-shaped peak having the local minimum value do not exist inthe region where the films are provided, the value at an inflectionpoint is accepted as the hydrogen concentration.

Various experiments can actually prove low off-state current of thetransistor including the highly-purified oxide semiconductor film as anactive layer. For example, even with an element with a channel width of1×10⁶ μm and a channel length of 10 μm, in the range of from 1 V to 10 Vof the voltage (drain voltage) between a source electrode and a drainelectrode, it is possible that off-state current (which is drain currentwhen the voltage between a gate electrode and the source electrode isless than or equal to 0 V) is less than or equal to the measurementlimit of a semiconductor characterization system, that is, less than orequal to 1×10⁻¹³ A. In that case, it can be found that an off-statecurrent density corresponding to a value obtained by dividing theoff-state current by the channel width of the transistor is lower thanor equal to 100 zA/μm. Further, in the experiment, a capacitor and atransistor (the thickness of a gate insulating film was 100 nm) wereconnected to each other and a circuit in which electric charge flowinginto or from the capacitor was controlled by the transistor was used.When the highly-purified oxide semiconductor film was used as a channelformation region in the transistor, and the off-state current of thetransistor was measured from a change in the amount of electric chargeof the capacitor per unit time, it was found that in the case where thevoltage between the source electrode and the drain electrode of thetransistor was 3 V, lower off-state current of 10 zA/μm to 100 zA/μm wasable to be obtained. Therefore, in the semiconductor device relating toone embodiment of the present invention, the off-state current densityof the transistor including the highly-purified oxide semiconductor filmas an active layer can be lower than or equal to 100 zA/μm, preferablylower than or equal to 10 zA/μm, or more preferably lower than or equalto 1 zA/μm, depending on the voltage between the source electrode anddrain electrode. Accordingly, the transistor including thehighly-purified oxide semiconductor film as an active layer has muchlower off-state current than a transistor including silicon havingcrystallinity.

A transistor including a highly-purified oxide semiconductor showsalmost no temperature dependence of off-state current. This is becausean impurity serving as an electron donor (donor) in the oxidesemiconductor is removed and the oxide semiconductor is highly purified,so that a conductivity type is close to a substantially intrinsic typeand the Fermi level is located in the center of the forbidden band. Thisalso results from the fact that the oxide semiconductor has an energygap of greater than or equal to 3 eV and includes very few thermallyexcited carriers. In addition, the source electrode and the drainelectrode are in a degenerated state, which is also a factor for showingno temperature dependence. The transistor is mainly operated withcarriers which are injected from the degenerated source electrode to theoxide semiconductor, and it can be explained that there is notemperature dependence of an off-state current because there is notemperature dependence of carrier density.

As the oxide semiconductor, a quaternary metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor, a ternary metal oxide such asan In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor, or a binarymetal oxide such as an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, an In—Ga—O-basedoxide semiconductor, an In—O-based oxide semiconductor, a Sn—O-basedoxide semiconductor, and a Zn—O-based oxide semiconductor, and the likecan be used. Note that in this specification, for example, anIn—Sn—Ga—Zn—O-based oxide semiconductor means a metal oxide includingindium (In), tin (Sn), gallium (Ga), and zinc (Zn). There is noparticular limitation on the stoichiometric composition ratio. The aboveoxide semiconductor may include silicon.

Alternatively, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

According to one embodiment of the present invention, the supply of thepower supply voltage to basic blocks which do not contribute to acircuit configuration is stopped by a programming cell, whereby powerconsumption of a semiconductor integrated circuit can be suppressed.

The programming cell having the above structure can suppress degradationof a gate insulating film by tunneling current, as compared to aconventional programming cell using the EEPROM; therefore, asemiconductor device in which the number of times of rewriting data canincrease can be provided.

In the programming cell having the above structure, the operatingvoltage which is needed for writing of data on a connection state isapproximately determined by the operating voltage of the secondtransistor. Accordingly, compared to the conventional programming cellusing the EEPROM, a semiconductor device in which the above operatingvoltage can be reduced significantly and power consumption is reducedcan be provided.

Unlike a programming cell formed using the SRAM, the programming cellhaving the above structure holds data using a transistor havingextremely low off-state current; accordingly, a connection state can beheld to some extent even when the power supply voltage is not alwayssupplied to the programming cell. Therefore, a semiconductor device inwhich power consumption is reduced can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a structure of a semiconductor device.

FIGS. 2A to 2C each illustrate a circuit diagram of a programming cell.

FIG. 3 is a diagram illustrating an operation of a semiconductor device.

FIG. 4 is a diagram illustrating an operation of a semiconductor device.

FIG. 5 is a diagram illustrating an operation of a semiconductor device.

FIG. 6 is a diagram illustrating a structure of a semiconductor device.

FIG. 7 is a diagram illustrating a structure of a semiconductor device.

FIG. 8 is a diagram illustrating a structure of a semiconductor device.

FIGS. 9A and 9B are timing charts each illustrating an operation of asemiconductor device.

FIGS. 10A to 10C are views illustrating a method for manufacturing asemiconductor device.

FIGS. 11A and 11B are views illustrating a method for manufacturing asemiconductor device.

FIGS. 12A to 12C are views illustrating a method for manufacturing asemiconductor device.

FIG. 13 is a longitudinal cross-sectional view of an inverted staggeredtransistor which includes an oxide semiconductor.

FIG. 14 is an energy band diagram (schematic diagram) of a cross sectiontaken along line A-A′ in FIG. 13.

FIG. 15A illustrates an energy band diagram where a positive voltage(VG>0) is applied to a gate electrode (GE), and FIG. 15B illustrates anenergy band diagram where a negative voltage (VG<0) is applied to thegate electrode (GE).

FIG. 16 is a diagram illustrating relationships between a vacuum level,a work function of a metal (φ_(M)), and electron affinity (χ) of anoxide semiconductor.

FIGS. 17A to 17D are views each illustrating a structure of anelectronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bechanged in various ways without departing from the spirit and scope ofthe present invention. Accordingly, the present invention should not beconstrued as being limited to the description of the embodiments below.

Note that a semiconductor device of the present invention includes, inits category, various semiconductor integrated circuits formed usingsemiconductor elements such as microprocessors, image processingcircuits, controllers for semiconductor display devices, digital signalprocessor (DSPs), micro controllers, and the like. In addition, thesemiconductor device of the present invention also includes, in itscategory, various devices such as RF tags formed using the abovesemiconductor integrated circuits, semiconductor display devices, andthe like. The semiconductor display devices include the following in itscategory: liquid crystal display devices, light-emitting devices inwhich a light-emitting element typified by an organic light-emittingelement (OLED) is provided for each pixel, electronic paper, digitalmicromirror devices (DMDs), plasma display panels (PDPs), field emissiondisplays (FEDs), and other semiconductor display devices in which asemiconductor element is included in a driver circuit.

(Embodiment 1)

In this embodiment, a structure and operation of a semiconductor deviceaccording to one embodiment of the present invention will be described.

A structure of a semiconductor device according to one embodiment of thepresent invention is illustrated in FIG. 1, as an example. Thesemiconductor device illustrated in FIG. 1 is provided with nine basicblocks A to I, programming cells SwAB to SwIH configured to controlconnections between the basic blocks A to I, and programming cells SwVAto SwVI configured to control the supply of a power supply potential VDDto the basic blocks A to I.

Note that in FIG. 1, a programming cell configured to control aconnection between an output terminal of the basic block A and an inputterminal of the basic block B is denoted as SwAB, for example. On theother hand, a programming cell configured to control a connectionbetween an output terminal of the basic block B and an input terminal ofthe basic block A is denoted as SwBA.

In FIG. 1, in order to avoid complexity of description, the case inwhich each of the nine basic blocks is connected to the basic block ofup, down, left, or right via the programming cells is illustrated.However, one embodiment of the present invention is not limited to thisstructure, and a designer can set the number of basic blocks and thestructure of the connection, as appropriate.

In FIG. 1, each basic block is connected via the programming cell to anode to which the power supply potential VDD is applied; however, inpractice, not only the power supply potential VDD but also a fixedpotential which is different from the power supply potential VDD, suchas a ground potential, are applied to each basic block. That is, whenthe power supply potential VDD is applied to the basic blocks, thepotential difference between the power supply potential VDD and theabove fixed potential is supplied to the basic blocks as power supplyvoltage.

In FIG. 1, the case where one output terminal of a given basic block isconnected to one input terminal of another basic block via theprogramming cell is described. However, one embodiment of the presentinvention is not limited to this structure. One output terminal of agiven basic block may be connected to a plurality of input terminals ofanother basic block via programming cells.

Note that there is no particular limitation on the structure of a logiccircuit used as the basic block. A logic circuit which performs a simplelogic operation such as an inverter, an AND, a NAND, or a NOR; an adder;a multiplier; a memory (e.g., a DRAM or the SRAM); or various arithmeticunits can be used as the basic block.

Each programming cell includes at least a first transistor (Tr1) whichcontrols a connection between two nodes and a second transistor (Tr2)which controls the supply of a potential to a gate electrode of thefirst transistor. Specifically, structural examples of the programmingcells are illustrated in FIGS. 2A to 2C.

FIG. 2A is an example of a programming cell having the simpleststructure, and the programming cell includes the first transistor (Tr1)which controls a connection between two nodes and the second transistor(Tr2) which controls the supply of a potential to a gate electrode ofthe first transistor (Tr1). Specifically, a source electrode of thefirst transistor (Tr1) is connected to a node N1 and a drain electrodeof the first transistor (Tr1) is connected to a node N2. Further, one ofthe source electrode (denoted as S) and the drain electrode (denoted asD) of the second transistor (Tr2) is connected to the gate electrode ofthe first transistor (Tr1), and a potential of a signal (Sig1) forcontrolling switching of the first transistor (Tr1) is applied to theother of the source electrode (denoted as S) and the drain electrode(denoted as D) of the second transistor (Tr2).

Note that a capacitor for holding a potential of the gate electrode ofthe first transistor (Tr1) may be provided.

When the second transistor (Tr2) is turned on in accordance with asignal input to a gate electrode of the second transistor (Tr2), apotential of a signal for controlling switching of the first transistor(Tr1) is applied to the gate electrode of the first transistor (Tr1).The first transistor (Tr1) performs switching in accordance with thepotential applied to the gate electrode of the first transistor (Tr1).When the first transistor (Tr1) is on, the node N1 is connected to thenode N2. In contrast, when the first transistor (Tr1) is off, the nodeN1 is not connected to the node N2, and one of these nodes is in a highimpedance state.

Next, when the second transistor (Tr2) is turned off in accordance witha signal input to the gate electrode of the second transistor (Tr2), thegate electrode of the first transistor (Tr1) is in a floating state, andthe potential is held. Accordingly, a connection state between the nodeN1 and the node N2 is held.

Note that “connection” in this specification refers to electricalconnection and corresponds to the state in which current, voltage, orpotential can be applied or transmitted. Therefore, the state ofconnection does not always mean a state of direct connection, butincludes in its category a state of indirect connection via a circuitelement such as a wiring, a resistor, a diode, or a transistor, in whichcurrent, voltage, or potential can be supplied or transmitted.

Note also that even when a circuit diagram illustrates independentcomponents which are connected to each other, there is the case whereone conductive film has functions of a plurality of components such asthe case where part of a wiring also functions as an electrode. The“connection” in this specification includes in its category such a casewhere one conductive film has functions of a plurality of components.

The names of the “source electrode” and the “drain electrode” includedin the transistor interchange with each other depending on the polarityof the transistor or difference between the levels of potentials appliedto the respective electrodes. In general, in an n-channel transistor, anelectrode to which a lower potential is applied is called a sourceelectrode, and an electrode to which a higher potential is applied iscalled a drain electrode. Further, in a p-channel transistor, anelectrode to which a lower potential is applied is called a drainelectrode, and an electrode to which a higher potential is applied iscalled a source electrode. In this specification, for convenience,although the connection relation of the transistor is described assumingthat the source electrode and the drain electrode are fixed in somecases; however, actually, the names of the source electrode and thedrain electrode interchange with each other depending on the relationbetween the above potentials.

Note that in the case of using a programming cell for controlling aconnection between the basic blocks, the node N1 is connected to aninput terminal or an output terminal of one basic block, and the node N2is connected to an input terminal or an output terminal of another basicblock. In the case of using a programming cell for controlling thesupply of the power supply voltage to the basic block, the basic blockis connected to one of the node N1 and the node N2, and the power supplypotential VDD is applied to the other of the node N1 and the node N2.

Next, an example of another programming cell is illustrated in FIG. 2B.The programming cell illustrated in FIG. 2B includes the firsttransistor (Tr1) which controls a connection between two nodes, thesecond transistor (Tr2) which controls the supply of a potential to thegate electrode of the first transistor (Tr1), and a third transistor(Tr3) which controls a connection between one of the above two nodes anda node to which a fixed potential VSS is applied.

Specifically, the source electrode of the first transistor (Tr1) isconnected to the node N1 and the drain electrode of the first transistor(Tr1) is connected to the node N2. One of the source electrode and thedrain electrode of the second transistor (Tr2) is connected to the gateelectrode of the first transistor (Tr1), a potential of a signal forcontrolling switching of the first transistor (Tr1) is applied to theother of the source electrode and the drain electrode of the secondtransistor (Tr2). One of the source electrode and the drain electrode ofthe third transistor (Tr3) is connected to the node N2, and the other isconnected to the node to which the fixed potential VSS is applied. Thegate electrode of the first transistor (Tr1) is connected to a gateelectrode of the third transistor (Tr3). Note that the first transistor(Tr1) is an n-channel transistor, and the third transistor (Tr3) is ap-channel transistor.

The programming cell illustrated in FIG. 2B includes a capacitor Cs forholding a potential of the gate electrode of the first transistor (Tr1).Although the capacitor Cs is not necessarily provided, when thecapacitor Cs is provided, a connection state between the node N1 and thenode N2 can be held for a longer period of time. Specifically, one ofthe pair of electrodes of the capacitor Cs is connected to the gateelectrode of the first transistor (Tr1), and the other is connected tothe node to which the fixed potential VSS is applied.

When the second transistor (Tr2) is turned on in accordance with asignal input to the gate electrode of the second transistor (Tr2), apotential of a signal for controlling switching of the first transistor(Tr1) is applied to the gate electrode of the first transistor (Tr1),the gate electrode of the third transistor (Tr3), and one electrode ofthe capacitor Cs. The first transistor (Tr1) performs switching inaccordance with the potential of the signal applied to the gateelectrode of the first transistor (Tr1). Then, the third transistor(Tr3) performs switching which is opposite to that of the firsttransistor (Tr1) because the polarity of the third transistor (Tr3) isdifferent from that of the first transistor (Tr1).

For example, when the first transistor (Tr1) is turned on, the thirdtransistor (Tr3) is turned off. As a result, the node N1 is connected tothe node N2. In contrast, when the first transistor (Tr1) is turned off,the third transistor (Tr3) is turned on. As a result, the node N1 is notconnected to the node N2, and the node N2 is connected to the node towhich the fixed potential VSS is applied.

Next, when the second transistor (Tr2) is turned off in accordance witha signal input to the gate electrode of the second transistor (Tr2), thegate electrode of the first transistor (Tr1) and the gate electrode ofthe third transistor (Tr3) are in a floating state, and the potential ofthe gate electrodes is held in the capacitor Cs. Accordingly, aconnection state between the node N1 and the node N2 and a connectionstate between the node N2 and the node to which the fixed potential VSSis applied are held.

Note that in FIG. 2B, a p-channel transistor is used for the thirdtransistor (Tr3); however, when a p-channel type is not generallyobtained similarly to an oxide semiconductor, or with a very lowmobility. Therefore, a resistor R can be used instead of a p-channeltransistor, as illustrated in FIG. 2C.

In one embodiment of the present invention, in the programming cellsillustrated in FIGS. 2A to 2C, off-state current or leakage current ofthe second transistor (Tr2) is extremely low. Specifically, a channelformation region of the second transistor (Tr2) includes a semiconductormaterial whose band gap is wider than that of a silicon semiconductorand whose intrinsic carrier density is lower than that of silicon. Whena semiconductor material having such characteristics is included in achannel formation region, a transistor with extremely low off-statecurrent can be realized.

As an example of the semiconductor material whose band gap is wider thanthat of a silicon semiconductor and whose intrinsic carrier density islower than that of silicon, a compound semiconductor such as siliconcarbide (SiC) or gallium nitride (GaN), an oxide semiconductor formedusing a metal oxide such as zinc oxide (ZnO), or the like can be used.Of these, the oxide semiconductor is advantageous in that it can beformed by a sputtering method or a wet method (printing method or thelike) and it is superior in mass production. If silicon carbide andgallium nitride do not form a single crystal, silicon carbide andgallium nitride do not obtain satisfactory characteristics. The processtemperature of silicon carbide for forming a single crystal isapproximately 1500° C., and the process temperature of gallium nitridefor forming a single crystal is approximately 1100° C. However, the filmformation temperature of the oxide semiconductor is as low as 300° C. to500° C. (at the maximum, approximately 700° C.), and a semiconductorelement formed using an oxide semiconductor can be stacked over anintegrated circuit formed using a semiconductor material such as singlecrystal silicon. Further, the oxide semiconductor can accompany anincrease in the size of a substrate. Accordingly, of the abovesemiconductor with a wide band-gap, particularly, the oxidesemiconductor has an advantage of high mass production efficiency. Inaddition, a crystalline oxide semiconductor having a superiorperformance (e.g., field-effect mobility) can be easily obtained by aheat treatment comprised between 450° C. and 800° C.

When an oxide semiconductor is used for the second transistor (Tr2), itis preferable that the above oxide semiconductor have a band gap ofgreater than or equal to 3 eV, and a carrier density of less than10¹²/cm³, preferably, less than 10¹¹/cm³. Specifically, the hydrogenconcentration in the above oxide semiconductor, which is measured bySIMS, is less than or equal to 5×10¹⁹/cm³, preferably less than or equalto 5×10¹⁸/cm³, more preferably less than or equal to 5×10¹⁷/cm³, stillmore preferably less than or equal to 1×10¹⁶/cm³. When an oxidesemiconductor having the above characteristics is used, off-statecurrent or leakage current of the second transistor (Tr2) can bereduced.

The gate insulating film of the second transistor (Tr2) preferably has athickness of greater than or equal to 10 nm, more preferably greaterthan or equal to 100 nm. The channel length of the second transistor(Tr2) is preferably greater than or equal to 100 nm, more preferablygreater than or equal to 300 nm. When the above structure is adopted,off-state current or leakage current of the second transistor (Tr2) canbe reduced.

In this way, when the second transistor (Tr2) with remarkably lowoff-state current or remarkably low leakage current is used, the supplyof a potential to the gate electrode of the first transistor iscontrolled, whereby the gate voltage of the first transistor is held fora long time. Therefore, the connection state between the node N1 and thenode N2 can also be held for a long time.

In order to further reduce off-state current or leakage current of thesecond transistor (Tr2) when the connection state is held, the potentialof the gate electrode of the second transistor (Tr2) may be set to bethe lowest among the programming cells. Specifically, it is preferablethat the potential of the gate electrode of the second transistor (Tr2)be lower than any other potentials by greater than or equal to 3 V, morepreferably greater than or equal to 5 V among potentials of electrodesand terminals included in all circuit elements which form theprogramming cells such as a transistor, a capacitor, or a resistor.

A channel formation region of the first transistor (Tr1) or the thirdtransistor (Tr3) may include a semiconductor material whose band gap iswider than that of a silicon semiconductor and whose intrinsic carrierdensity is lower than that of silicon in a manner similar to that of thesecond transistor (Tr2), or unlike the second transistor (Tr2), thechannel formation region of the first transistor (Tr1) or the thirdtransistor (Tr3) may include a semiconductor material which has highermobility. As examples of a semiconductor material having high mobility,germanium, silicon, and silicon germanium which have crystallinity suchas polycrystalline or single crystal, and single crystal silicon carbidecan be given. The first transistor (Tr1) or the third transistor (Tr3)may be formed using a semiconductor thin film, or may be formed using abulk semiconductor substrate.

In the former case, since the first transistor (Tr1) or the thirdtransistor (Tr3) has low off-state current or low leakage currentsimilar to that of the second transistor (Tr2), power consumption of asemiconductor device can be further reduced. In particular, in the caseof using the programming cell which controls the supply of the powersupply potential VDD to the basic block, a field-effect mobility of thefirst transistor (Tr1) of approximately 10 cm²V⁻¹s⁻¹ is sufficient foroperation and not so high operating speed is required. Off-state currentor leakage current generated in the programming cell which controls thesupply of the power supply potential VDD to the basic block directlyleads to an increase in power consumption. Therefore, in the case ofusing the programming cell which controls the supply of the power supplypotential VDD to the basic block, the former structure is preferablyadopted.

In the latter case, the mobility of the first transistor (Tr1)increases; therefore, resistance between the nodes connected with thefirst transistor (Tr1) can be reduced. In particular, in the case ofusing the programming cell for controlling a connection between thebasic blocks, high connection resistance between the basic blocks leadsto a decrease in operating speed of a semiconductor device. Therefore,in the case of using the programming cell for controlling a connectionbetween the basic blocks, the latter structure is preferably adopted.Note that when the channel formation region of the first transistor(Tr1) includes a semiconductor material which has higher mobility, thethickness of the gate insulating film is preferably greater than orequal to 10 nm in order to reduce off-state current or leakage current.

Note that when each channel formation region of the second transistor(Tr2), the first transistor (Tr1), and the third transistor (Tr3) hasthe same semiconductor material, the first transistor (Tr1) and thethird transistor (Tr3) may be formed in the same layer as the secondtransistor (Tr2). In that case, the thickness of each of the gateinsulating film of the first transistor (Tr1) and the third transistor(Tr3) is the same as that of the second transistor (Tr2) in design;however, the channel length or the channel width of the first transistor(Tr1) and the third transistor (Tr3) may be different from that of thesecond transistor (Tr2). For example, when the channel formation regionof the first transistor (Tr1) is desired to have low resistance, thechannel width may be twice or more as large as that of the secondtransistor (Tr2), preferably five or more times as large as that of thesecond transistor (Tr2).

Further, the first transistor (Tr1) may be formed by a non-self-alignedmethod. By the non-self-aligned method, although parasitic capacitanceoccurs due to overlap of the gate electrode and the source electrode orthe drain electrode, not so high speed operation is required for thefirst transistor (Tr1), and the above parasitic capacitance does notbecome a problem. Instead, the above parasitic capacitance functions asthe capacitor (Cs) which holds the potential of the gate electrode ofthe first transistor (Tr1). On the other hand, the parasitic capacitanceof the second transistor (Tr2) is preferably low in order to preventgeneration of a change in the potential of the gate electrode of thefirst transistor (Tr1) in switching.

Even when the same semiconductor material as that of the transistorwhich forms the basic block is used for the first transistor (Tr1), thethickness of the gate insulating film is preferably larger than that ofthe transistor which forms the basic block in order to reduce leakagecurrent of the first transistor (Tr1). In that case, the firsttransistor (Tr1) may be formed by the non-self-aligned method, and aconductive film formed in a layer which is different from that of thetransistor used for the basic block may be used as the gate electrode.

Note that in FIGS. 2A to 2C, the first transistor (Tr1), the secondtransistor (Tr2), and the third transistor (Tr3) each have the gateelectrode only on one side of the semiconductor film. However, oneembodiment of the present invention is not limited to this structure,and the above transistors may have a back gate electrode which isopposite to the side of the gate electrode with the semiconductor filminterposed therebetween. In that case, the back gate electrode may beelectrically insulated to be in a floating state, or may be in a statewhere the back gate electrode is supplied with a potential. In thelatter case, the gate electrode may be electrically connected to theback gate electrode, or a fixed potential such as a ground potential maybe always applied only to the back gate electrode. The level of thepotential applied to the back gate electrode is controlled, wherebythreshold voltage of the transistor can be controlled.

The programming cell which can be used for the semiconductor device isnot limited to the circuit configuration illustrated in FIGS. 2A to 2C.The programming cell may include at least the first transistor forcontrolling a connection between two nodes and the second transistor forcontrolling the supply of the potential to the gate electrode of theabove first transistor, and the channel formation region of the secondtransistor may include a semiconductor material such as an oxidesemiconductor having a band gap which is approximately three times aslarge as that of silicon.

Next, an operation of a semiconductor device when a plurality of basicblocks is arbitrarily connected using programming cells having the abovestructure will be described.

In the semiconductor device illustrated in FIG. 1, a connection betweenall the basic blocks is cut, and the supply of the power supply voltageto all the basic blocks is stopped. From the state illustrated in thisFIG. 1, a state in which the basic blocks are connected when some of theprogramming cells are on is illustrated in FIG. 3. Specifically, in FIG.3, when the programming cell SwAB, the programming cell SwBC, theprogramming cell SwCF, and the programming cell SwFI are turned on, theoutput terminal of the basic block A is connected to the input terminalof the basic block B, the output terminal of the basic block B isconnected to an input terminal of the basic block C, an output terminalof the basic block C is connected to an input terminal of the basicblock F, and an output terminal of the basic block F is connected to aninput terminal of the basic block I. When the programming cell SwVA, theprogramming cell SwVB, the programming cell SwVC, the programming cellSwVF, and the programming cell SwVI are turned on, the power supplypotential VDD is applied to the basic block A, the basic block B, thebasic block C, the basic block F, and the basic block I. Then, all theprogramming cells except the above programming cells are off. As aresult, as illustrated in FIG. 3, a path of a signal from an inputterminal (IN) of a semiconductor device to the basic block A, the basicblock B, the basic block C, the basic block F, and the basic block I isformed. Then, a first signal processing is performed in each of theabove basic blocks, whereby a signal is output from an output terminal(OUT).

In the state illustrated in FIG. 3, when a second signal processing isperformed after the first signal processing is performed on the inputsignal and an output signal is obtained, a connection between the basicblocks is changed. The change of the connection between the basic blocksis preferably performed after the all the programming cells are cutonce.

An example of a connection state between the basic blocks when thesecond signal processing is performed is illustrated in FIG. 4.Specifically, in FIG. 4, when the programming cell SwAD, the programmingcell SwDE, the programming cell SwEH, and the programming cell SwHI areturned on, the output terminal of the basic block A is connected to aninput terminal of the basic block D, an output terminal of the basicblock D is connected to an input terminal of the basic block E, anoutput terminal of the basic block E is connected to an input terminalof the basic block H, and an output terminal of the basic block H isconnected to the input terminal of the basic block I. When the elementSwVA, the programming cell SwVD, the programming cell SwVE, theprogramming cell SwVH, and the programming cell SwVI are turned on, thepower supply potential VDD is applied to the basic block A, the basicblock D, the basic block E, the basic block H, and the basic block I.Then, all the programming cells except the above programming cells areoff. As a result, as illustrated in FIG. 4, a path of a signal from theinput terminal (IN) of a semiconductor device to the basic block A, thebasic block D, the basic block E, the basic block H, and the basic blockI is formed. Then, the second signal processing is performed in each ofthe above basic blocks, whereby a signal is output from the outputterminal (OUT).

Also in the case where a third signal processing is performed, it ispreferable to change a connection between the basic blocks after all theprogramming cells are cut once. An example of a connection state betweenthe basic blocks in performing the third signal processing isillustrated in FIG. 5. Specifically, in FIG. 5, when the programmingcell SwAD, the programming cell SwDG, the programming cell SwGH, theprogramming cell SwHE, the programming cell SwEF, and the programmingcell SwFI are turned on, the output terminal of the basic block A isconnected to the input terminal of the basic block D, the outputterminal of the basic block D is connected an input terminal of thebasic block G, an output terminal of the basic block G is connected tothe input terminal of the basic block H, the output terminal of thebasic block H is connected to the input terminal of the basic block E,the output terminal of the basic block E is connected to the inputterminal of the basic block F, and the output terminal of the basicblock F is connected to the input terminal of the basic block I. Whenthe programming cell SwVA, the programming cell SwVD, the programmingcell SwVG, the programming cell SwVH, the programming cell SwVE, theprogramming cell SwVF, and the programming cell SwVI are turned on, thepower supply potential VDD is applied to the basic block A, the basicblock D, the basic block E, the basic block F, the basic block G, thebasic block H, and the basic block I. Then, all the programming cellsexcept the above programming cells are off. As a result, as illustratedin FIG. 5, a path of a signal from the input terminal (IN) of asemiconductor device to the basic block A, the basic block D, the basicblock G, the basic block H, the basic block E, the basic block F, andthe basic block I is formed. Then, the third signal processing isperformed in each of the above basic blocks, whereby a signal is outputfrom the output terminal (OUT).

Note that the time required for performing a configuration process(switching of the programming cells is controlled to form a circuit) issmall enough to be ignored, as long as a circuit configuration asillustrated in FIG. 3 is used, depending on the size of a matrix formedusing the basic blocks and the programming cells. For example, the abovetime needed per basic blocks of one row is less than or equal to 100nanoseconds. If a matrix formed using the basic blocks of three rows asillustrated FIG. 1 is used, the above time is less than 1 microsecond.

In one embodiment of the present invention, the supply of the powersupply voltage to the basic block which does not contribute to thecircuit configuration is stopped, so that power consumption of asemiconductor device can be reduced. In particular, in a semiconductordevice having a channel length of less than or equal to 50 nm, thethickness of a gate insulating film is less than or equal to severalnanometers, and leakage current flowing through the gate insulating filmaccounts for substantial amount of power consumption; therefore, theabove structure is effective in decreasing power consumption.

In one embodiment of the present invention, when the connection betweenthe basic blocks is controlled by a transistor with extremely lowoff-state current or extremely low leakage current, leakage current oroff-state current which flows between the basic block to which the powersupply voltage is applied and the basic block to which the power supplyvoltage is not applied can be reduced, and power consumption of asemiconductor device can be reduced.

Degradation of the gate insulating film in the programming cell havingthe above structure due to tunneling current can be reduced whencompared to that in a conventional programming cell formed using theEEPROM; therefore, a semiconductor device in which the number of timesof rewriting data can increase can be provided.

In the programming cell having the above structure, the operatingvoltage which is needed for writing of data is approximately determinedby the operating voltage of the second transistor (Tr2). Accordingly, asemiconductor device in which the above operating voltage can besignificantly reduced and power consumption can be suppressed ascompared to the conventional programming cell formed using the EEPROMcan be provided.

Unlike a programming cell formed using the SRAM, the programming cellhaving the above structure can hold a connection state to some extenteven when the power supply voltage is not always supplied to theprogramming cell. In addition, data is held using a transistor withremarkably low off-state current. Therefore, a semiconductor device inwhich power consumption can be reduced can be provided.

(Embodiment 2)

In Embodiment 1, the example in which a given basic block can beconnected to up to four adjacent basic blocks is described; however, inthis embodiment, an example in which a given basic block is connected toeight adjacent basic blocks will be described.

In a manner similar to that of FIG. 1, a semiconductor deviceillustrated in FIG. 6 is provided with nine basic blocks A to I,programming cells which control connections between the basic blocks Ato I, and programming cells which control the supply of the power supplypotential VDD to each of the basic blocks A to I.

Unlike FIG. 1, in the semiconductor device illustrated in FIG. 6, thecentral basic block E can be connected to all the basic blocks exceptthe basic block E, via programming cells SwAE to SwIE and programmingcells SwEA to SwEI.

Note that in FIG. 6, only the programming cells SwAE to SwIE whichcontrol connections between the central basic block E and the otherbasic blocks, the programming cells SwEA to SwEI which controlconnections between the central basic block E and the other basicblocks, and the programming cell SwVE which controls the supply of thepower supply potential VDD to the basic block E are illustrated.However, connections between the basic block A and the basic block B,between the basic block B and the basic block C, between the basic blockC and the basic block F, between the basic block F and the basic blockI, between the basic block A and the basic block D, between the basicblock D and the basic block G, between the basic block G and the basicblock H, and between the basic block H and the basic block I can becontrolled by the programming cells in a manner similar to that ofFIG. 1. In addition, the supply of the power supply potential VDD to thebasic block A, the basic block B, the basic block C, the basic block D,the basic block F, the basic block G, the basic block H, and the basicblock I can be controlled by the programming cells in a manner similarto that of FIG. 1.

Note that the structure described in Embodiment 1 can be used for theprogramming cells.

As illustrated in FIG. 6, the number of basic blocks which can beconnected to a given basic block can be set by a designer, asappropriate.

This embodiment can be implemented by being combined as appropriate withthe above embodiment.

(Embodiment 3)

A semiconductor device in which a large matrix is formed using basicblocks will be described using an example which is more generalized.

In this embodiment, attention is paid on a basic block U(n, m) of then-th row and the m-th column included in the semiconductor device. Asillustrated in FIG. 7, around the basic block U(n, m), there are eightbasic blocks: U(n−1, m−1); U(n, m−1); U(n+1, m−1); U(n−1, m); U(n+1, m);U(n−1, m+1); U(n, m+1); and U(n+1, m+1). The basic block U(n, m) isconnected to these eight basic blocks via programming cells in order totransmit and receive signals.

In FIG. 8, an example of a specific structure of a programming cell forcontrolling connections between the basic blocks illustrated in FIG. 7and a programming cell for controlling the supply of the power supplypotential VDD to each basic block is illustrated.

Note that an example in which the programming cell illustrated in FIG.2A is used is described in FIG. 8; however, any of the programming cellsillustrated in FIGS. 2B and 2C may be used. The structure of theprogramming cell is not limited to the circuit configurationsillustrated in FIGS. 2A to 2C. The structure of the programming cell mayinclude at least a first transistor for controlling a connection betweentwo nodes and a second transistor for controlling the supply of apotential to a gate electrode of the above first transistor and achannel formation region of the second transistor may include asemiconductor material such as an oxide semiconductor having a band gapwhich is approximately three times as large as that of silicon.

In FIG. 8, a transistor which is used for the programming cell and whosechannel formation region includes a semiconductor material such as anoxide semiconductor is illustrated using a symbol in which a circle isadded to a normal transistor symbol in order to be distinguished fromanother transistor.

The semiconductor device illustrated in FIG. 8 includes signal lines forcontrolling an operation of the programming cells as well as the abovebasic blocks.

Specifically, an example, in which five data signal lines Y1, Y2, Y3,Y4, and Y5 per one column and two scan signal lines X1 and X2 per onerow in a matrix are used as signal lines, is illustrated in FIG. 8;however, a structure in which one scan signal line per one row and ninedata signal lines per one column are used may be employed. When thenumber of scan signal lines is reduced, the time for performing aconfiguration process can be shortened.

Note that in FIG. 8, five data signal lines of the (n−1)-th column areY1 _(n−1), Y2 _(n−1), Y3 _(n−1), Y4 _(n−1), and Y5 _(n−1), whereas fivedata signal lines of the n-th column are Y1 _(n), Y2 _(n), Y3 _(n), Y4_(n), and Y5 _(n). In addition, two scan signal lines of the (m−1)-throw are X1 _(m−1) and X2 _(m−1), whereas two scan signal lines of them-th row are X1 _(m) and X2 _(m).

In order to reduce the total number of signal lines, the number of scansignal lines and data signal lines may be set in accordance with thebasic blocks which form a matrix. For example, as illustrated in FIG. 8,when five data signal lines per one column and two scan signal lines perone row are provided, approximately (2N+5M) signal lines are needed intotal for the semiconductor device including the basic blocks of N rowsand M columns. On the other hand, when nine data signal lines per onecolumn and one scan signal line per one row are used, approximately(N+9M) signal lines are needed in total for the semiconductor deviceincluding the basic blocks of N rows and M columns. Accordingly, if asemiconductor device where the relation N<4M is satisfied is used, theadoption of the former structure rather than the latter structure canreduce the total number of signal lines. On the other hand, if asemiconductor device where N>4M is satisfied is used, the adoption ofthe latter structure rather than the former structure can reduce thetotal number of signal lines.

In the semiconductor device described in this embodiment, a power supplypotential is supplied using the data signal line Y5. The data signalline Y5 (Y5 ₁, Y5 ₂, . . . , Y5 _(n−1), Y5 _(n), . . . ) transmits asignal for controlling switching of a first transistor 801 included ineach basic block to the first transistor 801 included in the programmingcells for supplying a power supply potential. Then, in a manner similarto that of another data signal line and another scan signal line, asignal is not needed to transmit to the data signal line Y5 in a periodin which the configuration process is not performed. Accordingly, inthat period, a power supply potential can be supplied to each basicblock from the data signal line Y5.

Note that in FIG. 8, the example in which only one input terminal andonly one output terminal are provided for each basic block is described;however, a basic block including a plurality of input terminals or aplurality of output terminals may be used.

Next, an example of an operation of the semiconductor device illustratedin FIG. 8 is described. A timing chart of a signal input to each signalline of the semiconductor device illustrated in FIG. 8 is shown in FIG.9A.

As illustrated in FIG. 9A, signals each having a pulse are sequentiallyapplied to the scan signal lines X1 _(m−1), X2 _(m−1), X1 _(m), and X2_(m) so as to have different timing. In FIG. 9A, only a potential of asignal applied to the scan signal line X1 _(m−1) is represented by asolid line, and a potential of a signal applied to the scan signal linesX2 _(m−1), X1 _(m), and X2 _(m) is represented by a dotted line. In FIG.9A, the potential of a signal is kept to be negative in a period otherthan a period in which a positive potential pulse is applied.Accordingly, when a signal having a pulse is applied to each scan signalline, the second transistor whose gate electrode is connected to thescan signal line is turned on.

Then, in accordance with timing when a pulse is applied to each scansignal line, selection signals are applied to the data signal lines Y1_(n−1), Y2 _(n−1), Y3 _(n−1), Y4 _(n−1), Y5 _(n−1), Y1 _(n), Y2 _(n), Y3_(n), Y4 _(n), and Y5 _(n). The selection signal applied to each datasignal line is input to a gate electrode of the first transistorprovided at an intersection of the scan signal line and the data signalline via the second transistor which is turned on. Then, in accordancewith a potential of this selection signal, switching of the above firsttransistor is controlled.

For example, in order to connect the basic blocks illustrated in FIG. 8in order of U(n−1, m−1), U(n, m), U(n, m+1), U(n−1, m), U(n, m−1), andU(n+1, m), a pulse may be applied to each signal line in accordance withthe timing chart illustrated in FIG. 9A.

Specifically, first, a pulse is applied to the data signal line Y4_(n−1) in accordance with timing when a pulse is applied to the scansignal line X1 _(m−1). Then, the first transistor at an intersection ofthe scan signal line X1 _(m−1) and the data signal line Y4 _(n−1) isturned on. The first transistor is a switch which controls a connectionbetween an output terminal of the basic block U(n−1, m) and an inputterminal of the basic block U(n, m−1). Accordingly, when the above firsttransistor is turned on, a path of a signal from the basic block U(n−1,m) to the basic block U(n, m−1) is formed.

Next, pulses are applied to the data signal lines Y2 _(n−1), Y5 _(n−1),Y2 _(n), and Y5 _(n) in accordance with timing when a pulse is appliedto the scan signal line X2 _(m−1); then, the first transistors atrespective intersections are turned on. Of these, the first transistorat the intersection of the scan signal line X2 _(m−1) and the datasignal line Y2 _(n−1) is a switch which controls a connection between anoutput terminal of the basic block U(n−1, m−1) and an input terminal ofthe basic block U(n, m). In addition, the first transistor at theintersection of the scan signal line X2 _(m−1) and the data signal lineY5 _(n−1) is a switch which controls the supply of a power supplypotential to the basic block U(n, m), and the first transistor at theintersection of the scan signal line X2 _(m−1) and the data signal lineY5 _(n) is a switch which controls the supply of a power supplypotential to the basic block U(n+1, m). Further, the first transistor atthe intersection of the scan signal line X2 _(m−1) and the data signalline Y2 _(n) is a switch which controls a connection between an outputterminal of the basic block U(n, m−1) and an input terminal of the basicblock U(n+1, m). Accordingly, when the above first transistors areturned on, a path of a signal from the basic block U(n−1, m−1) to thebasic block U(n, m) and a path of a signal from the basic block U(n,m−1) to the basic block U(n+1, m) are formed. Further, a power supplypotential is supplied to the basic blocks U(n, m) and U(n+1, m).

Next, a pulse is applied to the data signal line Y2 _(n−1) in accordancewith timing when a pulse is applied to the scan signal line X1 _(m).Then, the first transistor at an intersection of the scan signal line X1_(m) and the data signal line Y2 _(n−1) is turned on. The firsttransistor is a switch which controls a connection between an outputterminal of the basic block U(n, m+1) and an input terminal of the basicblock U(n−1, m). Accordingly, when the above first transistor is turnedon, a path of a signal from the basic block U(n, m+1) to the basic blockU(n−1, m) is formed.

Next, pulses are applied to the data signal lines Y5 _(n−1) and Y1 _(n)in accordance with timing when a pulse is applied to the scan signalline X2 _(m); then, the first transistors at respective intersectionsare turned on. Of these, the first transistor at the intersection of thescan signal line X2 _(m) and the data signal line Y1 _(n) is a switchwhich controls a connection between an output terminal of the basicblock U(n, m) and an input terminal of the basic block U(n, m+1). Thefirst transistor at the intersection of the scan signal line X2 _(m) andthe data signal line Y5 _(n−1) is a switch which controls the supply ofa power supply potential to the basic block U(n, m+1). Accordingly, whenthe above first transistor is turned on, a path of a signal from thebasic block U(n, m) to the basic block U(n, m+1) is formed and a supplypath of a power supply potential to the basic block U(n, m+1) is formed.

When the above operations are performed, the path of a signal from thebasic block U(n−1, m−1) to the basic block U(n, m), the path of a signalfrom the basic block U(n, m) to the basic block U(n, m+1), the path of asignal from the basic block U(n, m+1) to the basic block U(n−1, m), thepath of a signal from the basic block U(n−1, m) to the basic block U(n,m−1), and the path of a signal from the basic block U(n, m−1) to thebasic block U(n+1, m) are formed, and the supply paths of power supplypotentials to the basic blocks U(n, m+1), U(n, m), and U(n+1, m) areformed.

Although not illustrated in the timing chart of FIG. 9A, supply paths ofpower supply potentials to the basic blocks U(n−1, m−1), U(n, m−1), andU(n−1, m) besides the basic blocks U(n, m+1), U(n, m), and U(n+1, m) areformed.

As described above, a connection between the basic blocks and the supplyof a power supply potential are set by scanning of each scan signalline; therefore, as the number of scan signal lines increases, a longertime for setting is required. Note that the scan time per one scansignal line, namely, the time when a pulse is applied to the scan signalline, is less than or equal to 100 nanoseconds. Therefore, even when asemiconductor device includes 1000 scan signal lines, the time when allthe scan signal lines are scanned is less than 1 millisecond.

As described above, after a connection between the basic blocks and thesupply of a power supply potential are set, a power supply potential issupplied to the data signal line Y5 (Y5 ₁, Y5 ₂, . . . , Y5 _(n−1), Y5_(n), . . . ). A power supply potential is supplied from the above datasignal line to the basic block provided with the supply path of a powersupply potential via the first transistor; therefore, an arithmeticprocess can be performed.

Next, an operation of the semiconductor device illustrated in FIG. 8will be described using another example. In FIG. 9B, a timing chart of asignal input to each signal line of the semiconductor device illustratedin FIG. 8 is shown.

As illustrated in FIG. 9B, signals each having a pulse are sequentiallysupplied to the scan signal lines X1 _(m−1), X2 _(m−1), X1 _(m), and X2_(m) so as to have different timing. Also in FIG. 9B, only a potentialof a signal applied to the scan signal line X1 _(m−1) is illustrated bya solid line, and potentials of signals applied to the scan signal linesX2 _(m−1), X1 _(m), and X2 _(m) are illustrated by dotted lines. In FIG.9B, the potential of a signal is also kept to be negative in a periodother than a period in which a positive potential pulse is applied.Accordingly, when a signal having a pulse is applied to each scan signalline, the second transistor whose gate electrode is connected to thescan signal line is turned on.

In FIG. 9B, a pulse is not applied to a data signal line in a period inwhich a pulse is applied to the scan signal line X1 _(m−1). Next, apulse is applied to the data signal lines Y5 _(n−1), Y3 _(n), and Y4_(n) in accordance with timing when a pulse is applied to the scansignal line X2 _(m−1); then, the first transistors at respectiveintersections are turned on.

Of these, the first transistor at the intersection of the scan signalline X2 _(m−1) and the data signal line Y5 _(n−1) is a switch whichcontrols the supply of a power supply potential to the basic block U(n,m). Further, the first transistor at the intersection of the scan signalline X2 _(m−1) and the data signal line Y3, is a switch which connectsthe output terminal of the basic block U(n, m−1) and an input terminalof the basic block U(n+1, m−1). The first transistor at the intersectionof the scan signal line X2 _(m−1) and the data signal line Y4 _(n) is aswitch which connects an output terminal of the basic block U(n+1, m−1)and the input terminal of the basic block U(n, m). When the above firsttransistors are turned on, a path of a signal from the basic block U(n,m−1) to the basic block U(n+1, m−1) and a path of a signal from thebasic block U(n+1, m−1) to the basic block U(n, m) are formed, and asupply path of a power supply potential to the basic block U(n, m) isformed.

Next, a pulse is applied to the data signal line Y3 _(n−1) in accordancewith timing when a pulse is applied to the scan signal line X1 _(m);then, the first transistor is turned on at an intersection of the scansignal line X1 _(m) and the data signal line Y3 _(n−1). The firsttransistor is a switch which controls a connection between the outputterminal of the basic block U(n, m) and the input terminal of the basicblock U(n−1, m). Accordingly, when the above first transistor is turnedon, a path of a signal from the basic block U(n, m) to the basic blockU(n−1, m) is formed.

Next, a pulse is applied to the data signal line Y1 _(n−1) in accordancewith timing when a pulse is applied to the scan signal line X2 _(m);then, the first transistor at the intersection of the scan signal lineX2 _(m) and the data signal line Y1 _(n−1) is turned on. The firsttransistor at the intersection is a switch which controls a connectionof the output terminal of the basic block U(n−1, m) and an inputterminal of the basic block U(n−1, m+1). As described above, a path of asignal from the basic block U(n−1, m) to the basic block U(n−1, m+1) canbe formed.

When the above operations are performed, a path of a signal from thebasic block U(n, m−1) to the basic blocks U(n+1, m−1), U(n, m), U(n−1,m), and U(n−1, m+1) in that order is formed, and the supply path of apower supply potential to the basic block U(n, m) is formed. Althoughnot illustrated in the timing chart of FIG. 9B, supply paths of powersupply potentials to the basic blocks U(n, m−1), U(n+1, m−1), and U(n−1,m) besides the basic block U(n, m) are formed.

This embodiment can be implemented by being combined as appropriate withany of the above embodiments.

(Embodiment 4)

In this embodiment, a method for manufacturing a second transistor whichis used for a programming cell and which is formed using an oxidesemiconductor will be described. Note that in this embodiment, themethod for manufacturing the second transistor using an oxidesemiconductor is described taking the programming cell in FIG. 2B as anexample; however, a programming cell having another circuitconfiguration can also be formed using the manufacturing methoddescribed in this embodiment.

A first transistor and a third transistor can be formed using a normalCMOS process in which germanium, silicon, silicon germanium, singlecrystal silicon carbide, or the like is used. Also, the first transistorand the third transistor may be formed using a semiconductor thin filmor a bulk semiconductor substrate. In this embodiment, the case wherethe second transistor formed using an oxide semiconductor is formed overthe first transistor and the third transistor which are formed using anSOI (Silicon on Insulator) substrate is given as an example, and amanufacturing method thereof is described.

First, as illustrated in FIG. 10A, a first transistor 501 and a thirdtransistor 502 are formed using an SOI substrate. Specifically, thefirst transistor 501 is an n-channel transistor formed using a singlecrystal silicon film 503 over a substrate 500 having an insulatingsurface, and the third transistor 502 is a p-channel transistor formedusing a single crystal silicon film 504 over the substrate 500 having aninsulating surface. A gate insulating film of each of the firsttransistor 501 and the third transistor 502 is 10 nm to 100 nm,inclusive, and a channel length of each of the first transistor 501 andthe third transistor 502 is 100 nm to 500 nm, inclusive. The channellength of the first transistor 501 may be different from that of thethird transistor 502.

Next, a second transistor is formed using an oxide semiconductor overthe first transistor 501 and the third transistor 502.

First, as illustrated in FIG. 10A, an insulating film 505, an insulatingfilm 506, and an insulating film 507 are sequentially formed so as tocover the first transistor 501 and the third transistor 502. Note thatin this embodiment, the example in which the first transistor 501 andthe third transistor 502 are covered with three films, the insulatingfilm 505, the insulating film 506, and the insulating film 507 isdescribed; however, the number of insulating films provided between thefirst transistor 501 and the second transistor and between the thirdtransistor 502 and the second transistor is not necessarily three, andone insulating film, two insulating films, or four or more insulatingfilms may be formed.

The insulating film 505, the insulating film 506, and the insulatingfilm 507 are formed using materials which can withstand a temperature ofheat treatment in a later manufacturing step. Specifically, it ispreferable to use silicon oxide, silicon nitride, silicon nitride oxide,silicon oxynitride, aluminum nitride, aluminum oxide, or the like forthe insulating film 505, the insulating film 506, and the insulatingfilm 507. Also, in this specification, an oxynitride compound denotes amaterial containing a higher quantity of oxygen than that of nitrogen,and a nitride oxide compound denotes a material containing a higherquantity of nitrogen than that of oxygen.

A surface of the insulating film 507 may be planarized by a chemicalmechanical polishing (CMP) method or the like.

Next, as illustrated in FIG. 10B, a gate electrode 601 and electrodes602 are formed over the insulating film 507.

The gate electrode 601 and the electrodes 602 can be formed to have asingle-layer structure or a stacked structure using one or moreconductive films using a metal material such as molybdenum, titanium,chromium, tantalum, tungsten, neodymium, or scandium; an alloy materialwhich contains any of these metal materials as its main component; or anitride which contains any of these metals. Note that aluminum or coppercan also be used as such metal materials if aluminum or copper canwithstand a temperature of heat treatment performed in a later process.Aluminum or copper is preferably combined with a refractory metalmaterial so as to prevent a heat resistance problem and a corrosionproblem. As the refractory metal material, molybdenum, titanium,chromium, tantalum, tungsten, neodymium, scandium, or the like can beused.

For example, as a two-layer structure of the gate electrode 601 and theelectrodes 602, the following structures are preferable: a two-layerstructure in which a molybdenum film is stacked over an aluminum film, atwo-layer structure in which a molybdenum film is stacked over a copperfilm, a two-layer structure in which a titanium nitride film or atantalum nitride film is stacked over a copper film, and a two-layerstructure in which a titanium nitride film and a molybdenum film arestacked. As a three-layer structure of the gate electrode 601 and theelectrodes 602, the following structure is preferable: a stackedstructure containing an aluminum film, an alloy film of aluminum andsilicon, an alloy film of aluminum and titanium, or an alloy film ofaluminum and neodymium in a middle layer and any of a tungsten film, atungsten nitride film, a titanium nitride film, and a titanium film in atop layer and a bottom layer.

Further, a light-transmitting oxide conductive film of indium oxide, analloy of indium oxide and tin oxide (In₂O₃—SnO₂, abbreviated to ITO), analloy of indium oxide and zinc oxide, zinc oxide, zinc aluminum oxide,zinc aluminum oxynitride, zinc gallium oxide, or the like can be used asthe gate electrode 601 and the electrodes 602.

The thickness of each of the gate electrode 601 and the electrodes 602is in the range of 10 nm to 400 nm, preferably 100 nm to 200 nm. In thisembodiment, after the conductive film for the gate electrode is formedto have a thickness of 150 nm formed by a sputtering method using atungsten target, the conductive film is processed (patterned) into adesired shape by etching, whereby the gate electrode 601 and theelectrodes 602 are formed. Note that when end portions of the formedgate electrode are tapered, coverage with a gate insulating film stackedthereover is improved, which is preferable. Note that a resist mask maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

Next, as illustrated in FIG. 10C, a gate insulating film 603 is formedover the gate electrode 601 and the electrodes 602. The gate insulatingfilm 603 is formed using a film having a single layer or a stacked layerwhich includes a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, an aluminumnitride oxide film, a hafnium oxide film, or a tantalum oxide film by aplasma enhanced CVD method, a sputtering method, or the like. It ispreferable that the gate insulating film 603 include impurities such asmoisture or hydrogen as little as possible. In the case where a siliconoxide film is formed by a sputtering method, a silicon target or aquartz target is used as a target and oxygen or a mixed gas of oxygenand argon is used as a sputtering gas.

The oxide semiconductor that is made to be an i-type oxide semiconductoror a substantially i-type oxide semiconductor (highly-purified oxidesemiconductor) by removal of impurities is extremely sensitive to aninterface state and an interface electric charge; thus, characteristicsof an interface between the highly-purified oxide semiconductor and thegate insulating film 603 are important. Therefore, the gate insulatingfilm (GI) that is in contact with the highly-purified oxidesemiconductor needs to be of high quality.

For example, a high-density plasma enhanced CVD using a microwave(frequency: 2.45 GHz) is preferably used, in which case an insulatingfilm which is dense, has high withstand voltage, and is of high qualitycan be formed. The highly-purified oxide semiconductor and thehigh-quality gate insulating film are in close contact with each other,whereby density of interface states can be reduced and favorableinterface characteristics can be obtained.

Needless to say, a different deposition method such as a sputteringmethod or a plasma enhanced CVD method can be used as long as ahigh-quality insulating film can be formed as a gate insulating film. Inaddition, any insulating film can be used as long as film quality andcharacteristics of an interface with an oxide semiconductor are modifiedby heat treatment performed after deposition. In any case, anyinsulating film that has a reduced interface state density of aninterface between a gate insulating film and the oxide semiconductor andcan form a favorable interface as well as having a favorable filmquality as the gate insulating film can be used.

The gate insulating film 603 may have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film having low proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film are stacked. In that case, theinsulating film such as a silicon oxide film or a silicon oxynitridefilm is formed between the insulating film having a high barrierproperty and the oxide semiconductor film. As the insulating film havinga high barrier property, a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, an aluminum nitride oxide film, and thelike can be given, for example. The insulating film having a highbarrier property is used, so that impurities in an atmosphere, such asmoisture or hydrogen, or impurities included in the substrate, such asan alkali metal or a heavy metal, can be prevented from entering theoxide semiconductor film, the gate insulating film 603, or the interfacebetween the oxide semiconductor film and another insulating film and thevicinity thereof. In addition, the insulating film having low proportionof nitrogen such as a silicon oxide film or a silicon oxynitride film isformed so as to be in contact with the oxide semiconductor film, so thatthe insulating film having a high barrier property can be prevented frombeing in contact with the oxide semiconductor film directly.

For example, a stacked-layer film with a thickness of 100 nm may beformed as the gate insulating film 603 as follows: a silicon nitridefilm (SiN_(y) (y>0)) with a thickness of 50 nm to 200 nm, inclusive isformed by a sputtering method as a first gate insulating film, and asilicon oxide film (SiO_(x) (x>0)) with a thickness of 5 nm to 300 nm,inclusive is stacked over the first gate insulating film as a secondgate insulating film. The thickness of the gate insulating film 603 maybe set as appropriate depending on characteristics needed for atransistor, and may be approximately 350 nm to 400 nm.

In this embodiment, the gate insulating film 603 having a structure inwhich a silicon oxide film having a thickness of 100 nm formed by asputtering method is stacked over a silicon nitride film having athickness of 50 nm formed by a sputtering method is formed.

Note that the gate insulating film is in contact with the oxidesemiconductor. When hydrogen enters the oxide semiconductor,characteristics of the transistor are adversely affected; therefore, itis preferable that the gate insulating film do not contain hydrogen,hydroxyl, and moisture. In order that the gate insulating film 603contains as little hydrogen, hydroxyl, and moisture as possible, it ispreferable that an impurity adsorbed on the substrate 500, such asmoisture or hydrogen, be eliminated and removed by preheating thesubstrate 500, over which the gate electrode 601 and the electrodes 602are formed, in a preheating chamber of a sputtering apparatus, as apretreatment for film formation. The temperature for the preheating is100° C. to 400° C., inclusive, preferably, 150° C. to 300° C.,inclusive. As an exhaustion unit provided in the preheating chamber, acryopump is preferable. Note that this preheating treatment can beomitted.

Next, over the gate insulating film 603, an oxide semiconductor filmhaving a thickness of 2 nm to 200 nm, inclusive, preferably 3 nm to 50nm, inclusive, more preferably 3 nm to 20 nm, inclusive is formed. Theoxide semiconductor film is formed by a sputtering method using an oxidesemiconductor target. Moreover, the oxide semiconductor film can beformed by a sputtering method under a rare gas (e.g., argon) atmosphere,an oxygen atmosphere, or a mixed atmosphere of a rare gas (e.g., argon)and oxygen.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust attached to a surface of the gate insulating film 603 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of the voltage to a target side, anRF power source is used for application of the voltage to a substrateside under an argon atmosphere to generate plasma in the vicinity of thesubstrate to modify a surface. Note that instead of an argon atmosphere,a nitrogen atmosphere, a helium atmosphere, or the like may be used.Alternatively, an argon atmosphere to which oxygen, nitrous oxide, orthe like is added may be used. Further alternatively, an argonatmosphere to which chlorine, carbon tetrafluoride, or the like is addedmay be used.

As described above, as the oxide semiconductor film, the following oxidesemiconductors can also be used: a quaternary metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor; a ternary metal oxide such asan In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor; a binarymetal oxide such as an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, an In—Ga—O-basedoxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-basedoxide semiconductor; and a Zn—O-based oxide semiconductor. The aboveoxide semiconductor may include silicon.

Alternatively, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-basedoxide semiconductor thin film with a thickness of 30 nm, which isobtained by a sputtering method using a target including indium (In),gallium (Ga), and zinc (Zn), is used. As the above target, for example,a target having a composition ratio of metals, In:Ga:Zn=1:1:0.5,In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. The target may containSiO₂ at 2 wt % to 10 wt %, inclusive. The filling rate of the targetincluding In, Ga, and Zn is 90% to 100%, inclusive, preferably 95% to99.9%, inclusive. With the use of the target with high filling rate, adense oxide semiconductor film is formed.

In this embodiment, the oxide semiconductor film is formed over thesubstrate 500 as follows: the substrate is held in the chamber withpressure reduced, residual moisture in the chamber is removed, asputtering gas from which hydrogen and moisture are removed isintroduced, and the above target is used. At that time, the substratemay be heated at 100° C. to 600° C., inclusive, preferably 200° C. to400° C., inclusive. Film formation is performed while the substrate isheated, whereby the concentration of an impurity contained in the oxidesemiconductor film formed can be reduced. In addition, damage due tosputtering can be reduced. In order to remove residual moisture in thetreatment chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the deposition chamber which is evacuated with thecryopump, for example, a hydrogen atom, a compound containing a hydrogenatom, such as water (H₂O), (more preferably, also a compound containinga carbon atom), and the like are removed, whereby the concentration ofan impurity contained in the oxide semiconductor film formed in thedeposition chamber can be reduced.

As one example of the deposition conditions, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of the oxygen flow rate is 100%). Note that apulsed direct-current (DC) power supply is preferable because dustgenerated in deposition can be reduced and the film thickness can bemade uniform.

In order that the oxide semiconductor film contains as little hydrogen,hydroxyl, and moisture as possible, it is preferable that an impurityadsorbed on the substrate 500, such as moisture or hydrogen, beeliminated and removed by preheating the substrate 500, on which theprocess up to and including the step of forming the gate insulating film603 is already performed, in a preheating chamber of a sputteringapparatus, as a pretreatment for film formation. The temperature for thepreheating is 100° C. to 400° C., inclusive, preferably, 150° C. to 300°C., inclusive. As an exhaustion unit provided in the preheating chamber,a cryopump is preferable. Note that this preheating treatment can beomitted. Further, this preheating may be similarly performed on thesubstrate 500 on which the process up to and including the step offorming a source electrode 608, a drain electrode 609, and wirings 610to 613 is already performed, before the formation of an insulating film614.

Next, as illustrated in FIG. 10C, the oxide semiconductor film isprocessed (patterned) into a desired shape by etching or the like,whereby an island-shaped oxide semiconductor film 605 is formed over thegate insulating film 603 to overlap with the gate electrode 601.

A resist mask for forming the island-shaped oxide semiconductor film 605may be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

Note that as the etching of the island-shaped oxide semiconductor film605, dry etching, wet etching, or both of them may be employed. As anetching gas for dry etching, a gas containing chlorine (chlorine-basedgas such as chlorine (Cl₂), boron trichloride (BCl₃), silicontetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused. Alternatively, a gas containing fluorine (a fluorine-based gassuch as carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)), hydrogen bromide (HBr),oxygen (O₂), any of these gases to which a rare gas such as helium (He)or argon (Ar) is added, or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into a desired shape, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, a mixed solution of phosphoric acid,acetic acid, and nitric acid, or the like can be used. Alternatively,ITO-07N (produced by KANTO CHEMICAL CO., INC.) may be used. The etchantafter the wet etching is removed together with the etched materials bycleaning. The waste liquid including the etchant and the material etchedoff may be purified and the material may be reused. When a material suchas indium contained in the oxide semiconductor film is collected fromthe waste liquid after the etching and reused, the resources can beefficiently used and cost can be reduced.

Note that it is preferable that reverse sputtering be performed beforethe formation of a conductive film in a subsequent step so that a resistresidue or the like that is attached to surfaces of the island-shapedoxide semiconductor film 605 and the gate insulating film 603 isremoved.

Note that it is found that the oxide semiconductor film formed bysputtering or the like includes large amount of impurities such asmoisture or hydrogen. Moisture and hydrogen easily form a donor leveland thus serve as impurities in the oxide semiconductor. Thus, in oneembodiment of the present invention, in order to reduce an impurity suchas moisture or hydrogen in the oxide semiconductor film, heat treatmentis performed on the oxide semiconductor film 605 under an atmosphere ofnitrogen, oxygen, ultra-dry air (air in which a water content is lessthan or equal to 20 ppm, preferably less than or equal to 1 ppm, morepreferably less than or equal to 10 ppb), or a rare gas (argon, helium,or the like).

When heat treatment is performed on the oxide semiconductor film 605,moisture or hydrogen in the oxide semiconductor film 605 can be reduced.Specifically, heat treatment may be performed at 300° C. to 700° C.,inclusive, preferably 300° C. to 500° C., inclusive. For example, heattreatment may be performed at 500° C. for approximately three minutes tosix minutes, inclusive. When an RTA method is used for the heattreatment, dehydration or dehydrogenation can be performed in a shorttime; therefore, treatment can be performed even at a temperature higherthan the strain point of a glass substrate.

In this embodiment, an electrical furnace that is one of heat treatmentapparatuses is used.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon is used.

Note that in the heat treatment, it is preferable that moisture,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. It is preferable that the purity of nitrogenor the rare gas such as helium, neon, or argon which is introduced intoa heat treatment apparatus be set to be higher than or equal to 6N(99.9999%), preferably higher than or equal to 7N (99.99999%) (that is,the impurity concentration is lower than or equal to 1 ppm, preferablylower than or equal to 0.1 ppm).

Through the above steps, the hydrogen concentration in the oxidesemiconductor film 605 can be reduced and the oxide semiconductor film605 can be highly purified. Thus, the oxide semiconductor film can bestabilized. In addition, heat treatment at a temperature lower than orequal to the glass transition temperature makes it possible to form anoxide semiconductor film with a wide band gap in which carrier densityis extremely low. Therefore, a transistor can be manufactured using alarge-sized substrate, so that mass productivity can be increased. Inaddition, by using the oxide semiconductor film in which the hydrogenconcentration is reduced and purity is improved, it is possible tomanufacture a transistor with high withstand voltage and a high on-offratio. The above heat treatment can be performed at any time after theoxide semiconductor film is formed.

When the oxide semiconductor film is heated, plate-like crystals areformed on the surface thereof depending on a material and heat conditionof the oxide semiconductor film in some cases. The plate-like crystalsare preferably single crystal bodies which are c-axis-aligned in adirection substantially perpendicular to a surface of the oxidesemiconductor film. Even if the plate-like crystals are not singlecrystal bodies, each crystal is preferably a polycrystalline body whichis c-axis-aligned in a direction substantially perpendicular to thesurface of the oxide semiconductor film. Further, it is preferable thatthe polycrystalline bodies be c-axis-aligned and that the a-b planes ofcrystals correspond, or the a-axis or the b-axis of the crystals bealigned with each other. Note that in the case where a surface of a baseof the oxide semiconductor film is uneven, each plate-like crystal is apolycrystal. Therefore, the surface of the base is preferably as even aspossible.

Next, the insulating film 505, the insulating film 506, the insulatingfilm 507, and the gate insulating film 603 are partly etched, wherebycontact holes are formed, the contact holes reaching a pair of impurityregions 606 included in the first transistor 501, a pair of impurityregions 607 included in the third transistor 502, a gate electrode 510included in the first transistor 501, and a gate electrode 511 includedin the third transistor 502.

Note that one of the pair of impurity regions 606 included in the firsttransistor 501 functions as a source electrode and the other thereoffunctions as a drain electrode. In addition, one of the pair of impurityregions 607 included in the third transistor 502 functions as a sourceelectrode and the other thereof functions as a drain electrode.

Then, a conductive film to be used for a source electrode and a drainelectrode (including a wiring formed using the same layer as the sourceelectrode and the drain electrode) is formed by a sputtering method or avacuum deposition method so as to cover the oxide semiconductor film605, and then the conductive film is patterned by etching or the like,whereby the source electrode 608, the drain electrode 609, and thewirings 610 to 613 are formed as illustrated in FIG. 11A.

Note that the source electrode 608 and the drain electrode 609 are incontact with the oxide semiconductor film 605. The wiring 610 is incontact with one of the pair of impurity regions 606. The wiring 611 isin contact with the other of the pair of impurity regions 606 and one ofthe pair of impurity regions 607. The wiring 612 is in contact with thegate electrode 510 and one of the electrodes 602. In addition, thewiring 613 is in contact with the gate electrode 511 and another one ofthe electrodes 602.

As a material of the conductive film to form the source electrode 608,the drain electrode 609, the wirings 610 to 613 (including anotherwiring formed in the same layer as the source electrode 608, the drainelectrode 609, and the wirings 610 to 613), there are an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy including any ofthese elements as a component; an alloy film including any of theseelements in combination; and the like. A structure in which a refractorymetal film of Cr, Ta, Ti, Mo, W, or the like is stacked on a lower sideand/or an upper side of a metal film of Al, Cu, or the like may be used.When an aluminum material to which an element for preventing generationof hillocks and whiskers in an aluminum film, such as Si, Ti, Ta, W, Mo,Cr, Nd, Sc, or Y, is added is used, heat resistance can be increased.

The conductive film may have a single-layer structure or a stackedstructure of two or more layers. For example, a single-layer structureof an aluminum film including silicon, a two-layer structure in which atitanium film is stacked over an aluminum film, a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in that order, and the like can be given.

The conductive film to be the source electrode 608, the drain electrode609, and the wirings 610 to 613 (including another wiring formed in thesame layer as the source electrode 608, the drain electrode 609, and thewirings 610 to 613) may be formed using a conductive metal oxide. As theconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an alloy of indium oxide and tin oxide, an alloy of indiumoxide and zinc oxide (In₂O₃—ZnO), or any of the metal oxide materialsincluding silicon or silicon oxide can be used.

In the case where heat treatment is performed after formation of theconductive film, the conductive film preferably has enough heatresistance to withstand the heat treatment.

Note that materials and etching conditions are adjusted as appropriateso that the oxide semiconductor film 605 is not removed as much aspossible in etching of the conductive film. Depending on etchingconditions, an exposed portion of the island-shaped oxide semiconductorfilm 605 may be partly etched, so that a groove (a recessed portion) isformed in some cases.

In this embodiment, a titanium film is used for the conductive film.Therefore, wet etching can be selectively performed on the conductivefilm using a solution (ammonia hydrogen peroxide mixture) containingammonia and hydrogen peroxide water; however, the oxide semiconductorfilm 605 is partly etched in some cases. In addition, as the ammoniahydrogen peroxide mixture, specifically, a solution in which hydrogenperoxide water of 31 wt %, ammonia water of 28 wt %, and water are mixedat a volume ratio of 5:2:2 is used. Alternatively, dry etching may beperformed on the conductive film using a gas containing chlorine (Cl₂),boron trichloride (BCl₃), or the like.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask which is a light-exposure maskthrough which light is transmitted so as to have a plurality ofintensities. A resist mask formed with the use of a multi-tone mask hasa plurality of thicknesses and further can be changed in shape byetching; therefore, the resist mask can be used in a plurality ofetching steps for processing into different patterns. Therefore, aresist mask corresponding to at least two kinds or more of differentpatterns can be formed by one multi-tone mask. Thus, the number oflight-exposure masks can be reduced and the number of correspondingphotolithography steps can be also reduced, whereby simplification of aprocess can be realized.

Next, plasma treatment is performed thereon, using a gas such as N₂O,N₂, or Ar. By the plasma treatment, water or the like which is adsorbedon an exposed surface of the oxide semiconductor film is removed. Plasmatreatment may be performed using a mixed gas of oxygen and argon aswell.

After the plasma treatment, as illustrated in FIG. 11B, the insulatingfilm 614 is formed so as to cover the source electrode 608, the drainelectrode 609, the wirings 610 to 613, and the oxide semiconductor film605. The insulating film 614 preferably includes impurities such asmoisture or hydrogen as little as possible, and the insulating film 614may be formed using a single-layer insulating film or a plurality ofinsulating films stacked. When hydrogen is contained in the oxideinsulating film 614, entry of the hydrogen to the oxide semiconductorfilm, or extraction of oxygen in the oxide semiconductor film byhydrogen may occur, thereby causing the backchannel portion of the oxidesemiconductor film to have lower resistance (to be n-type), so that aparasitic channel may be formed. Therefore, it is preferable that adeposition method in which hydrogen is not used is employed in order toform the insulating film 614 containing as little hydrogen as possible.A material having a high barrier property is preferably used for theabove insulating film 614. For example, as the insulating film having ahigh barrier property, a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, an aluminum nitride oxide film, or thelike can be used. When a plurality of insulating films stacked is used,an insulating film having lower proportion of nitrogen, such as asilicon oxide film or a silicon oxynitride film, is formed on the sidecloser to the oxide semiconductor film 605 than the above insulatingfilm having a high barrier property. Then, the insulating film having ahigh barrier property is formed so as to overlap with the sourceelectrode 608, the drain electrode 609, and the oxide semiconductor film605 with the insulating film having lower proportion of nitrogen betweenthe insulating film having a barrier property and the source electrode608, the drain electrode 609, and the oxide semiconductor film 605. Whenthe insulating film having a high barrier property is used, theimpurities such as moisture or hydrogen can be prevented from enteringthe oxide semiconductor film 605, the gate insulating film 603, or theinterface between the oxide semiconductor film 605 and anotherinsulating film and the vicinity thereof. In addition, the insulatingfilm having lower proportion of nitrogen such as a silicon oxide film ora silicon oxynitride film is formed so as to be in contact with theoxide semiconductor film 605, whereby the insulating film formed using amaterial having a high barrier property can be prevented from being incontact with the oxide semiconductor film 605 directly.

In this embodiment, the insulating film 614 having a structure in whicha silicon nitride film having a thickness of 100 nm formed by asputtering method is stacked over a silicon oxide film having athickness of 200 nm formed by a sputtering method is formed. Thesubstrate temperature in deposition may be room temperature to 300° C.,inclusive and in this embodiment, is 100° C.

Note that after the insulating film 614 is formed, heat treatment may beperformed. Heat treatment is performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is less than orequal to 20 ppm, preferably less than or equal to 1 ppm, more preferablyless than or equal to 10 ppb), or a rare gas (argon, helium, or thelike) preferably at 200° C. to 400° C., inclusive, for example, 250° C.to 350° C., inclusive. In this embodiment, heat treatment for 1 hour at250° C. under a nitrogen atmosphere is performed. Alternatively, RTAtreatment for a short time at a high temperature may be performed beforethe formation of the source electrode 608, the drain electrode 609, andthe wirings 610 to 613 in a manner similar to that of the heat treatmentperformed on the oxide semiconductor film. Even when oxygen deficiencyoccurs in the oxide semiconductor film 605 due to the heat treatmentperformed on the oxide semiconductor film, the insulating film 614containing oxygen is provided in contact with an exposed region of theoxide semiconductor film 605 provided between the source electrode 608and the drain electrode 609, and then heat treatment is performed,whereby oxygen is supplied to the oxide semiconductor film 605.Therefore, when oxygen is supplied to the region of the oxidesemiconductor film 605 which is in contact with the insulating film 614,oxygen deficiency serving as a donor can be reduced and thestoichiometric composition ratio can be satisfied. As a result, theoxide semiconductor film 605 can be made to be an i-type semiconductorfilm or a substantially i-type semiconductor film. Accordingly, electriccharacteristics of the transistor can be improved and variation in theelectric characteristics thereof can be reduced. There is no particularlimitation on timing for this heat treatment as long as it is performedafter formation of the insulating film 614. The heat treatment alsoserves as heat treatment in another process, for example, heat treatmentin forming a resin film or heat treatment for reducing resistance of atransparent conductive film, whereby the oxide semiconductor film 605can be made to be an i-type semiconductor film or a substantially i-typesemiconductor film without an increase in the number of fabricationsteps.

After a conductive film is formed over the insulating film 614, theconductive film is patterned, so that a back gate electrode may beformed in a position where the back gate electrode overlaps with theoxide semiconductor film 605. When the back gate electrode is formed, aninsulating film is formed so as to cover the back gate electrode. Theback gate electrode can be formed using a material and a structuresimilar to those of the gate electrode 601, the electrodes 602, thesource electrode 608 and the drain electrode 609, or the wirings 610 to613.

The thickness of the back gate electrode is set to be 10 nm to 400 nm,preferably 100 nm to 200 nm. For example, the back gate electrode may beformed in a such a manner that a conductive film in which a titaniumfilm, an aluminum film, and a titanium film are stacked is formed, aresist mask is formed by a photolithography method or the like, andunnecessary portions are removed by etching so that the conductive filmis processed (patterned) to a desired shape.

With the above steps, a second transistor 620 and a capacitor 623 areformed. Note that the capacitor 623 is formed in a region where thesource electrode 608 overlaps with still another one of the electrodes602 with the gate insulating film 603 interposed therebetween. Note thatthe capacitor 623 is not necessarily formed over the same layer as thesecond transistor 620, and for example, the capacitor 623 may be formedover the same layer as the first transistor 501 and the third transistor502.

The second transistor 620 includes the gate electrode 601, the gateinsulating film 603 over the gate electrode 601, the oxide semiconductorfilm 605 which overlaps with the gate electrode 601 over the gateinsulating film 603, and a pair of the source electrode 608 and thedrain electrode 609 formed over the oxide semiconductor film 605. Thesecond transistor 620 may include the insulating film 614 formed overthe oxide semiconductor film 605, as a component. The second transistor620 illustrated in FIG. 11B has a channel-etched structure in which partof the oxide semiconductor film 605 is exposed between the sourceelectrode 608 and the drain electrode 609.

Although the second transistor 620 is described as a single-gatetransistor, a multi-gate transistor including a plurality of channelformation regions can be manufactured when a plurality of the gateelectrodes 601 which is electrically connected is included if needed.

Next, as in this embodiment, how characteristics of the transistor areaffected by high purification of the oxide semiconductor film by removalof impurities such as moisture, hydrogen, or the like contained in theoxide semiconductor film as much as possible will be described.

FIG. 13 is a cross-sectional view of a transistor including an oxidesemiconductor. An oxide semiconductor film (OS) is provided over a gateelectrode (GE) with a gate insulating film (GI) therebetween; a sourceelectrode (S) and a drain electrode (D) are provided thereover; and aninsulating film is provided over the source electrode (S) and the drainelectrode (D).

FIG. 14 is an energy band diagram (schematic diagram) of a cross sectiontaken along line A-A′ in FIG. 13. In FIG. 14, a black circle (●)represents an electron and has an electric charge −q, whereas a whitecircle (∘) represents a hole and has an electric charge +q. Whenpositive voltage (VD>0) is applied to the drain electrode (D), the casewhere voltage (VG=0) is not applied to the gate electrode (GE) isindicated by dashed lines and the case where positive voltage (VG>0) isapplied to the gate electrode (GE) is indicated by solid lines. In thecase where voltage is not applied to the gate electrode (GE), carriers(electrons) are not injected to the oxide semiconductor (OS) film sidefrom the source electrode (S) due to a high potential barrier, showingan off state in which current does not flow. On the other hand, in thecase where positive voltage is applied to the gate electrode (GE), thepotential barrier is reduced, showing an on state in which current flowsto the oxide semiconductor (OS) film.

FIGS. 15A and 15B are energy band diagrams (schematic diagrams) of across section taken along line B-B′ in FIG. 13. FIG. 15A illustrates thecase where positive voltage (VG>0) is applied to a gate electrode (GE)and an on state where carriers (electrons) flow between the sourceelectrode and the drain electrode. FIG. 15B illustrates a state wherenegative voltage (VG<0) is applied to the gate electrode (GE), in otherwords, the case where the transistor is in an off-state.

FIG. 16 shows the relation between the vacuum level, the work function(φ_(M)) of metal, and the electron affinity (χ) of the oxidesemiconductor.

At normal temperature, electrons in the metal are degenerated and theFermi level is located in the conduction band. On the other hand, aconventional oxide semiconductor is an n-type semiconductor, and theFermi level (Ef) thereof is located nearer to the conduction band (Ec)away from an intrinsic Fermi level (Ei) which is located in the centerof the band gap. Note that it is known that part of hydrogen is a donorin the oxide semiconductor and is one factor causing an oxidesemiconductor to be an n-type semiconductor. Further, oxygen vacancy isknown as one of the causes to produce an n-type oxide semiconductor.

On the other hand, an oxide semiconductor in one embodiment of thepresent invention is an intrinsic (i-type) or a substantially intrinsicoxide semiconductor which is obtained by removing hydrogen that is ann-type impurity from an oxide semiconductor and purifying the oxidesemiconductor such that impurities other than a main component of theoxide semiconductor are prevented from being contained therein as muchas possible. That is, the oxide semiconductor becomes an i-typesemiconductor not by addition of impurities but by removal of impuritiessuch as moisture or hydrogen and oxygen vacancy as much as possible tohave high purity, so that an oxide semiconductor which is an intrinsic(i-type) semiconductor or is a substantially intrinsic (i-type)semiconductor is obtained. With the above structure, the Fermi level(Ef) can be substantially close to the same level as the intrinsic Fermilevel (Ei), as indicated by an arrow.

It is said that the band gap (Eg) of an oxide semiconductor is 3.15 eVand the electron affinity (χ) is 4.3 V. The work function of titanium(Ti) used for forming the source and drain electrodes is substantiallyequal to the electron affinity (χ) of the oxide semiconductor. In thatcase, a Schottky barrier to electrons is not formed at an interfacebetween the metal and the oxide semiconductor. There are materialssatisfying this condition other than titanium.

In that case, as illustrated in FIG. 15A, the electron moves along thelowest part of the oxide semiconductor, which is energetically stable,at an interface between the gate insulating film and the highly-purifiedoxide semiconductor.

In FIG. 15B, when negative voltage is applied to the gate electrode(GE), holes which are minority carriers are substantially zero;therefore, current is substantially close to zero.

In this manner, the oxide semiconductor film is highly purified so thatimpurities such as moisture or hydrogen other than a main component ofthe oxide semiconductor are contained as little as possible, whereby theoperation of the transistor can be favorable.

This embodiment can be implemented in combination with any of the aboveembodiments.

(Embodiment 5)

In this embodiment, a structure of a programming cell in which a secondtransistor including an oxide semiconductor film has a structure whichis different from that of Embodiment 4 will be described.

A programming cell illustrated in FIG. 12A includes the first transistor501 and the third transistor 502 similar to that of Embodiment 4. Abottom-gate second transistor 630 which has a channel protectivestructure and includes an oxide semiconductor film is formed over thefirst transistor 501 and the third transistor 502 in FIG. 12A.

The second transistor 630 includes a gate electrode 631 provided overthe insulating film 507, a gate insulating film 632 provided over thegate electrode 631, an oxide semiconductor film 633 which overlaps withthe gate electrode 631 over the gate insulating film 632, a channelprotective film 634 which is provided over the oxide semiconductor film633 to overlap with the gate electrode 631, and a source electrode 635and a drain electrode 636 which are provided over the oxidesemiconductor film 633. The second transistor 630 may include aninsulating film 637 provided over the oxide semiconductor film 633, as acomponent.

The channel protective film 634 can prevent the portion of the oxidesemiconductor film 633 serving as a channel formation region from beingdamaged in a later step (e.g., reduction in thickness due to plasma oran etchant in etching). Therefore, reliability of the transistor can beimproved.

The channel protective film 634 can be formed using an inorganicmaterial that contains oxygen (e.g., silicon oxide, silicon nitrideoxide, silicon oxynitride, aluminum oxide, or aluminum oxynitride). Thechannel protective film 634 can be formed by a vapor deposition methodsuch as a plasma enhanced CVD method or a thermal CVD method, or asputtering method. After the channel protective film 634 is formed, theshape thereof is processed by etching. Here, the channel protective film634 is formed in such a manner that a silicon oxide film is formed by asputtering method and processed by etching using a mask formed byphotolithography.

An inorganic material containing oxygen is used for the channelprotective film 634, whereby a structure can be provided, in whichoxygen is supplied to a region of the oxide semiconductor film 633 incontact with at least the channel protective film 634 and oxygendeficiency serving as a donor is reduced to satisfy the stoichiometriccomposition ratio even when the oxygen deficiency occurs in the oxidesemiconductor film 633 by heat treatment for reducing moisture orhydrogen. Accordingly, the channel formation region can be made to be ani-type channel formation region or a substantially i-type channelformation region, variation in electric characteristics of a transistordue to oxygen deficiency can be reduced, and electric characteristicscan be improved.

Note that the second transistor 630 may further include a back gateelectrode over the insulating film 637. The back gate electrode may beformed so as to overlap with a channel formation region in the oxidesemiconductor film 633. The back gate electrode may be electricallyinsulated to be in a floating state, or may be in a state where the backgate electrode is supplied with a potential. In the case of the latter,the back gate electrode may be supplied with the same potential as thegate electrode 631, or may be supplied with a fixed potential such as aground potential. The level of the potential supplied to the back gateelectrode may be controlled so as to control the threshold voltage ofthe second transistor 630.

A programming cell illustrated in FIG. 12B includes the first transistor501 using crystalline silicon and the third transistor 502 usingcrystalline silicon in a manner similar to that of Embodiment 4. Abottom-contact second transistor 640 which includes an oxidesemiconductor film is formed over the first transistor 501 and the thirdtransistor 502 in FIG. 12B.

The second transistor 640 includes a gate electrode 641 formed over theinsulating film 507, a gate insulating film 642 over the gate electrode641, a source electrode 643 and a drain electrode 644 which are over thegate insulating film 642, and an oxide semiconductor film 645 whichoverlaps with the gate electrode 641. The second transistor 640 mayinclude an insulating film 646 provided over the oxide semiconductorfilm 645, as a component.

The thickness of each of the source electrode 643 and the drainelectrode 644 of the bottom-contact second transistor 640 in FIG. 12B ispreferably smaller than that of the bottom-gate transistor described inEmbodiment 4 in order to prevent disconnection of the oxidesemiconductor film 645 formed later. Specifically, the thickness of eachof the source electrode 643 and the drain electrode 644 is 10 nm to 200nm, preferably 50 nm to 75 nm.

Note that the second transistor 640 may further include a back gateelectrode over the insulating film 646. The back gate electrode may beformed so as to overlap with a channel formation region in the oxidesemiconductor film 645. The back gate electrode may be electricallyinsulated to be in a floating state, or may be in a state where the backgate electrode is supplied with a potential. In the case of the latter,the back gate electrode may be supplied with the same potential as thegate electrode 641, or may be supplied with a fixed potential such as aground potential. The level of the potential supplied to the back gateelectrode may be controlled so as to control the threshold voltage ofthe second transistor 640.

A programming cell illustrated in FIG. 12C includes the first transistor501 and the third transistor 502 which are formed using crystallinesilicon in a manner similar to that of Embodiment 4. A top-gate secondtransistor 650 which includes an oxide semiconductor film is formed overthe first transistor 501 and the third transistor 502 in FIG. 12C.

The second transistor 650 includes a source electrode 651 and a drainelectrode 652 which are formed over the insulating film 507, an oxidesemiconductor film 653 formed over the source electrode 651 and thedrain electrode 652, a gate insulating film 654 over the oxidesemiconductor film 653, and a gate electrode 655 which overlaps with theoxide semiconductor film 653 over the gate insulating film 654. Inaddition, the second transistor 650 may include an insulating film 656provided over the gate electrode 655, as a component.

The thickness of each of the source electrode 651 and the drainelectrode 652 of the top-gate second transistor 650 in FIG. 12C ispreferably smaller than that of the bottom-gate transistor described inEmbodiment 4 in order to prevent disconnection of the oxidesemiconductor film 653 formed later. Specifically, the thickness of eachof the source electrode 651 and the drain electrode 652 is 10 nm to 200nm, preferably 50 nm to 75 nm.

This embodiment can be implemented in combination with any of the aboveembodiments.

EXAMPLE 1

A semiconductor device according to one embodiment of the presentinvention is used, so that a highly reliable electronic device and anelectronic device with low power consumption can be provided. Inparticular, in the case where a portable electronic device which hasdifficulty in continuously receiving electric power is used, asemiconductor device with low power consumption according to oneembodiment of the present invention is added to the component of thedevice, whereby an advantage in increasing the continuous duty periodcan be obtained. Further, by use of a transistor with low off-statecurrent, redundant circuit design which is needed to cover a failurecaused by high off-state current is unnecessary; therefore, the densityof an integrated circuit used for the semiconductor device can beincreased, and a higher performance semiconductor device can be formed.

The semiconductor device according to one embodiment of the presentinvention can be used for display devices, laptops, or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media and have displays fordisplaying the reproduced images such as digital versatile discs(DVDs)). Other than the above, as an electronic appliance which can usethe semiconductor device according to one embodiment of the presentinvention, mobile phones, game machines including portable gamemachines, portable information terminals, e-book readers, video cameras,digital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), vending machines, and thelike can be given. Specific examples of these electronic devices areillustrated in FIGS. 17A to 17D.

FIG. 17A illustrates a display device including a housing 7011, adisplay portion 7012, a supporting base 7013, and the like. Thesemiconductor device according to one embodiment of the presentinvention can be used for an integrated circuit for controlling drivingof the display device, so that a highly reliable display device and adisplay device with low power consumption can be provided. Note that adisplay device includes all display devices for displaying information,such as display devices for personal computers, for receiving televisionbroadcast, and for displaying advertisement, in its category.

FIG. 17B illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, an operation key 7037, a stylus 7038,and the like. The semiconductor device according to one embodiment ofthe present invention can also be used for an integrated circuit forcontrolling the driving of the portable game machine, so that a highlyreliable portable game machine and a portable game machine with lowpower consumption can be provided. Although the portable game machineillustrated in FIG. 17B includes two display portions 7033 and 7034, thenumber of display portions included in the portable game machine is notlimited to two.

FIG. 17C illustrates a mobile phone including a housing 7041, a displayportion 7042, an audio-input portion 7043, an audio-output portion 7044,operation keys 7045, a light-receiving portion 7046, and the like. Lightreceived in the light-receiving portion 7046 is converted intoelectrical signals, whereby external images can be loaded. Thesemiconductor device according to one embodiment of the presentinvention can also be used for an integrated circuit for controlling thedriving of the mobile phone, so that a highly reliable mobile phone anda mobile phone with low power consumption can be provided.

FIG. 17D is a portable information terminal including a housing 7051, adisplay portion 7052, an operation key 7053, and the like. In theportable information terminal illustrated in FIG. 17D, a modem may beincorporated in the housing 7051. The semiconductor device according toone embodiment of the present invention can also be used for anintegrated circuit for controlling the driving of the portableinformation terminal, so that a highly reliable portable informationterminal and a portable information terminal with low power consumptioncan be provided.

This example can be implemented by being combined as appropriate withany of the above embodiments.

This application is based on Japanese Patent Application serial no.2010-009569 filed with Japan Patent Office on Jan. 20, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: basicblocks each comprising a logic circuit; a node configured to be appliedwith a power supply potential; and programming cells each comprising: afirst transistor comprising a gate electrode, a source electrode and adrain electrode; and a second transistor comprising a gate electrode, asource electrode and a drain electrode, one of the source electrode andthe drain electrode of the second transistor being configured to be at asame potential as the gate electrode of the first transistor, whereinthe node is connected to one of the basic blocks through the sourceelectrode and the drain electrode of the first transistor of acorresponding one of the programming cells, and wherein the secondtransistors each comprise a channel formation region comprising an oxidesemiconductor layer.
 2. The semiconductor device according to claim 1,further comprising in each programming cell: a third transistorcomprising a gate electrode, a source electrode and a drain electrode,the gate electrode of the third transistor being connected to the gateelectrode of the first transistor, and one of the source electrode andthe drain electrode of the third transistor being connected to the otherof the source electrode and the drain electrode of the first transistor.3. The semiconductor device according to claim 1, further comprising ineach programming cell: a third transistor comprising a gate electrode, asource electrode and a drain electrode, the gate electrode of the thirdtransistor being connected to the gate electrode of the firsttransistor, and one of the source electrode and the drain electrode ofthe third transistor being connected to the other of the sourceelectrode and the drain electrode of the first transistor; and acapacitor comprising a first electrode and a second electrode, the firstelectrode being connected to the other of the source electrode and thedrain electrode of the third transistor, and the second electrode beingconnected to each of the gate electrode of the third transistor and thegate electrode of the first transistor.
 4. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor layer comprises anIn—Ga—Zn—O-based oxide semiconductor.
 5. The semiconductor deviceaccording to claim 1, wherein off-state current densities of the secondtransistors are lower than or equal to 100 zA/μm.
 6. The semiconductordevice according to claim 1, wherein the first transistors and thesecond transistors are formed on a same substrate, the secondtransistors being formed over an insulating film that covers the firsttransistors.
 7. A programmable logic device including the semiconductordevice according to claim
 1. 8. A semiconductor device comprising: basicblocks each comprising a logic circuit; and programming cells eachcomprising: a first transistor comprising a gate electrode, a sourceelectrode and a drain electrode; and a second transistor comprising agate electrode, a source electrode and a drain electrode, one of thesource electrode and the drain electrode of the second transistor beingconfigured to be at a same potential as the gate electrode of the firsttransistor, wherein a first one of the basic blocks is connected to asecond one of the basic blocks through the source electrode and thedrain electrode of one of the first transistors, and wherein the secondtransistors each comprise a channel formation region comprising an oxidesemiconductor layer.
 9. The semiconductor device according to claim 8,further comprising in each programming cell: a third transistorcomprising a gate electrode, a source electrode and a drain electrode,the gate electrode of the third transistor being connected to the gateelectrode of the first transistor, and one of the source electrode andthe drain electrode of the third transistor being connected to the otherof the source electrode and the drain electrode of the first transistor.10. The semiconductor device according to claim 8, further comprising ineach programming cell: a third transistor comprising a gate electrode, asource electrode and a drain electrode, the gate electrode of the thirdtransistor being connected to the gate electrode of the firsttransistor, and one of the source electrode and the drain electrode ofthe third transistor being connected to the other of the sourceelectrode and the drain electrode of the first transistor; and acapacitor comprising a first electrode and a second electrode, the firstelectrode being connected to the other of the source electrode and thedrain electrode of the third transistor, and the second electrode beingconnected to each of the gate electrode of the third transistor and thegate electrode of the first transistor.
 11. The semiconductor deviceaccording to claim 8, wherein the oxide semiconductor layer comprises anIn—Ga—Zn—O-based oxide semiconductor.
 12. The semiconductor deviceaccording to claim 8, wherein off-state current densities of the secondtransistors are lower than or equal to 100 zA/μm.
 13. The semiconductordevice according to claim 8, wherein the first transistors and thesecond transistors are formed on a same substrate, the secondtransistors being formed over an insulating film that covers the firsttransistors.
 14. A programmable logic device including the semiconductordevice according to claim
 8. 15. A semiconductor device comprising:basic blocks each comprising a logic circuit; a wiring; and programmingcells each comprising: a first transistor comprising a gate electrode, asource electrode and a drain electrode; and a second transistorcomprising a gate electrode, a source electrode and a drain electrode,one of the source electrode and the drain electrode of the secondtransistor being connected to the gate electrode of the firsttransistor, wherein the wiring is connected to the basic blocks throughthe source electrodes and the drain electrodes of the first transistors,and wherein the second transistors each comprise a channel formationregion comprising an oxide semiconductor layer.
 16. The semiconductordevice according to claim 15, further comprising in each programmingcell: a third transistor comprising a gate electrode, a source electrodeand a drain electrode, the gate electrode of the third transistor beingconnected to the gate electrode of the first transistor, and one of thesource electrode and the drain electrode of the third transistor beingconnected to the other of the source electrode and the drain electrodeof the first transistor.
 17. The semiconductor device according to claim15, further comprising in each programming cell: a third transistorcomprising a gate electrode, a source electrode and a drain electrode,the gate electrode of the third transistor being connected to the gateelectrode of the first transistor, and one of the source electrode andthe drain electrode of the third transistor being connected to the otherof the source electrode and the drain electrode of the first transistor;and a capacitor comprising a first electrode and a second electrode, thefirst electrode being connected to the other of the source electrode andthe drain electrode of the third transistor, and the second electrodebeing connected to each of the gate electrode of the third transistorand the gate electrode of the first transistor.
 18. The semiconductordevice according to claim 15, wherein the oxide semiconductor layercomprises an In—Ga—Zn—O-based oxide semiconductor.
 19. The semiconductordevice according to claim 15, wherein off-state current density of thesecond transistor is lower than or equal to 100 zA/μm.
 20. Thesemiconductor device according to claim 15, wherein the firsttransistors and the second transistors are formed on a same substrate,the second transistors being formed over an insulating film that coversthe first transistors.
 21. A programmable logic device including thesemiconductor device according to claim 15.